i have a 2.5 Warn,and have yarded alot of bikes out with it,,,the 2.5 has plenty of power,and if you get too crazy with size,your just adding undue weight to you bike
if i ever have a problem with sliding while im trying to pull someone out,i take my tow rope,and hook it to a tree,and to the rear of my bike,,,,,if a tree isnt handy,just get someone to pull their bike nose to nose to yours,and just let the winch do the work,,,,,i have yet to have a bike i couldnt yard out with the 2.5 yet