Can I use different size ram in a X58 board

If your timings are different it may cause some issues
but otherwise no problems using say one 1Gb stick and one 512Mb stick in the same channel if their same speed, even if they were two different speeds the system would just utilise both at the lower speed rating
Moto

yes you can.....only thing is that you wont get the dual channel activated....
http://www.hardwaresecrets.com/article/133