Most of the ones I have seen Cibie, Bosch etc are in the 6 - 7 inch diameter range. Square or oblong ones were in vogue at one point, again make by Cibie and even lucas!
It's a matter of taste, what looks right and what you need them for.
If your going to be doing seriously fast night time driving then 7 - 10 inch Cibie spot lights with 100 watt halogen bulbs are the way to go (too many night road rallies for me :o )
However word of warning - check they are legal in your part of the world and consider upgrading any wiring, preferabley with relays
Spot lights should only come on with main beam on (UK)
Fog lights can be on with both high and low beam, but here in Scotland a lot of folk drive with them on all the time, very annoying at times especially if they are not aimed correctly
Malc