Hi, just joined this forum today because I'm new to two things:

1) Eldar

2) 40K

I have done wood elves in fantasy but the eldar looked so cool that I bought the new book and warwalkers right away.

Another thing... what's a good size for a guardian unit?

greetings and welcome, the war walkers are very cool kits as to guardians i'll leave that answer to more knowledgeable people here

10 is the minimum, but it does depend. I'd say 15 and up for any guardian squad that you plan to charge forward without a weapons platform. (That includes Storm Guardians), and no more than 10 for any squad with a weapons platform. But it also dpends on what else you want to put into the force.

Welcome to the Craftworld!

For support units, 10 Guardians witha weapons platform is usually pretty decent, but if you plan for it to be a significant part of your firebase/line/etc, 15-20 is always pretty good.