im new to making pc's i need to know if all psu''s are the same size because if more watts bigger size right ? i dont know i have an atx case came with 350 so if i buy a 580 watt or more will it fit? thx

Not neccesarily. In most cases, they are the same size. (The Novia 400W and the Antec TrueControl 550W are the exact same size.) However, some high watt power supplies (OCZ PowerStream 600W, SilverStone Zeus 650W) are larger and perform better.

An atx psu >should< work in any atx case, but they don't always in the real world. You could measure your current psu and then compare it's size to power supplies you're considering. Here's a guide to psu's
http://www.hardforum.com/showthread.php?t=792566