How big of a back tire can I buy??

[gsJack]

is it easy to bend the brake rod and also is it safe to do it?

Yes and yes.      The brake rod (or brake strut as they call it on GS part fiches) is made of mild steel and bends rather easily without breaking.  Some have heated it to bend it but I bent it cold.  It was marked where the tire touched it so I just took it off and layed it on a piece of 2x4 and hit it with a 40oz hammer right on the spot where the tire marked it while holding it on the 2x4 with the other hand and the outer ends bent upwards so it curved nicely around the tire.