Lampshade size?

I generally trust my instincts completely, but it seems there might be actual rules for lampshades. The two lamp bases are rather gigantic, pure white ginger jar shapes. Quite dramatic, but I have chosen to place them in our bedroom, which is large but not huge. Hence a desire to minimize shade size so they don't end up dominating the space. (Sorry, no pics.) I've selected black shades, but they may be too short, as they expose the neck between the base and the bulb area. The neck is also black, though, so it doesn't look TOO bad. Not too bad was NOT the effect I was going for, however, and shades are pricey, even the ugly ones!