I have a 1957 TO35, I pull a mid weight 5 ft. Rotary cutter.
In heavy grass or tall thick weeds a 5 ft. Cutter is all my tractor needs.
If all you are going to do is clip a pasture then you should be able to handle a 6 ft rotary cutter.
Depending on the brand there can be significant difference in the weight of the cutter.
You do not want a cutter so heavy that each time you pick it up, the front of the tractor comes off the ground.