What size seedling cells?

Needing some new cell inserts for standard 11x22" planting trays, I checked some web sites and was amazed at the number of different sizes available: 12 cells per tray, 18, 24, 32, 36, 40, 48, 60, 72, and "plug cells" with more than 100 tray,

In the past I've used mostly the 48 cell (~1.5"x2.4"x2.3"deep) inserts, but I started thinking that if the manufacturers offer them, the other sizes must be good for something.

So, what are your recommendations on what size seedling cells are best for
a) tomatoes
b) peppers
c) other veggies
d) annuals
e) perennials ?

And are the "deep cells" (3.1" deep) better than the standard 2.3" depth?

(Just to be specific, I am talking about the cells used for individual seedlings under lights before transplanting outside, since tomatoes and tiny seeds have to be started separately and transplanted into these cells.)

DonShirer,
I know everyone has their preferences, I think a lot has to do with the number of plants you are planning on growing. Also the type of root system the plant has makes a difference, and weather or not the plants take to transplanting well or not.
I like to grow just a few tomatoes, a few peppers, etc. things in the vegetable line. So I like small containers, because the tomatoes grow much faster than the peppers the lights for each must be adjusted differently. I have found that my tomatoes do best in very shallow containers, my favorite being egg cartons, very shallow. Naturally after they get the first set of true leaves they must be transplanted very early, but that actually seems to help the tomatoes.
Peppers take forever it seems to germinate and then they grow very slowly, and I have not tried transplanting them much, mainly because they grow so slowly they are normally ready for the garden by the time they outgrow their original container.
For some reason I have had better luck with flower seed getting them started in flats than in small cells. Also I grow a much larger number of flowers than vegetables so putting 50 or so seeds in a flat is much easier.
For plants like Lupines that put down a deep tap root a deep container is preferred, I actually used an empty 1ga.plus 1pt. plastic ice cream container for the extra depth and they did great, I just dug down deep to get the entire tap root, trans planted them when they were very tiny, but the roots were still pretty long.
If I am planting Zinnias or Marigolds then I like the larger count cells, even the 48 or 72 count. I plant a lot of them for my garden, so I like to have a full tray. That way they take up less room and all the plants in the tray can receive the same lighting conditions.
So my favorites depend on what I am planting, they each have a purpose, I guess that is why they make them all.

To save room under my lights, I mostly use the smallest cells that seem practical: 72 per tray or 128 per tray.

Those tiny "128 cell trays" mean you have to pot the seedlings up pretty soon after they have any real leaves. That means I have to find room for 100 4" pots, which is another problem.

if I want to leave a plant in its original cell more than a few weeks after emerging, I use some "deep" 50-cell trays I found.

I should stress that I am NOT expert. But bigger cells give you more margin for error.

P.S.

"Insert trays" tend to have very flimsy, tear-apart six-packs or 4-packs. The smallest cells I've seen that way are 72-cell-insert-trays with 12 6-packs each.

Plug trays are much sturdier, but they're all-on-piece.

When I buy the 50, 128 or 200 cell "plug trays" or "propagation trays", I cut the tray up lengthwise into 4-5 blocks.
Like 4 blocks of 8x4 cells, or 5 blocks of 10x4 cells.
The 50-cell-plug-tray I cut into two 5x3 blocks and one 5x4 block (10 rows)

Narrower than that tends to tip over.
Wider than that means too many cells of all-one-plant.

The 50-cell tray could probably be just two 5x5 blocks but I still find that unwieldy to transplant from.

I use tin snips or narrow, sharp scissors to cut as far in from each edge as I can without splitting or tearing the plastic, then use a sharp knife to extend the cuts to reach each other. Sometimes you have to insert the scissors as far as you can before snipping one cells-width, to prevent skewing the cut left or right. Take many small snips.

It might have been smart to get out a soldering iron or craft heating tool with a narrow blade and melt them apart, but it can be done with scissors. If you hack up one cell, you can either skip using that cell, or reinforce it by nesting a whole cell inside the damaged one.

P.P.S.

If you find that the 18, 24 or 32-cell trays have cells that are too wide and shallow for one plant, you can still use them as mini-flats. Broadcast-sow 10-40 seeds in one "big" cell and wait for them to emerge. Then prick them out as early as possible and put the best single seedlings into smaller cells, Dixie cups, or 4" pots.

This is one way to start vast numbers of plants of many different varieties. It's easier to label each "mini-flat" with one plant name than it is to divide up one 10"x21" flat and label each of 18, 24 or 32 areas with a different plant name!

(I use "stakes" made from plastic mini-blind slats marked with a 0.9 mm or 0.7 mm mechanical pencil. Often I cut the slats to half-width and sometimes 1/3rd-width for labeling rows in a 128-c ell-tray. They also hold plastic film up off the soil surface during germination.)

One thing about a "mini-flat" that is really a "big, shallow cell" ... it's small enough that you can dump the whole cell out all at once and untangle the roots easier than doing the same thing with a 10" x 21" big traditional flat.