What MIG wire diameter is recommended?

It depends on how thick the material is that needs to be welded, but in reality the .030” wire diameter is probably the most versatile.  With an .030” wire you have the ability to weld thin sheet up to thick plate (with multiple passes).  The .030” wire also gives the welding machine a larger wire feed speed range to optimize the arc characteristics.  Some people will argue the .035” wire is more versatile and this is true if you are using a 200A+ machine, but on a machine that’s less than 200A, go with .030”.