It depends on the tiller. My brother had a 4' King Kutter with 6 tines per row and I have a 62" Bushhog with 4 tines per row. His 4' was harder to pull than my 62".
If it helps he had a B7500 which was suppose to be 21hp but the tag on the engine said it was 24hp. His tiller was all his tractor would handle but on the other hand it handled my 62" fine. Bottom line, if you get a 6 tine I would go with a 48". If you get a 4 tine you'll probably be fine with a 60".
Of course the type soil you have a tilling depth should be factored in before you make your choice.