What is upsizing on tires and how high can i go?

I have a 2001 Ford f150 van and it has 235 75 15 NOW

What can i safely upsize too?

There about a 5 inch gap right now between the wheel wells is that enough space?

It all depends, check with a local tire shop. Most should have a limit on the sizes your allowed to go. If you always have your tires done at one of these shops, its highly advised to use what tires they say are for your vehicle.

For example if you should buy some tires from some hole in the wall tire repair shop & they put non authorized tires on your vehicle & then next month you run over something & destroy the tire. If you can't get back to that person for whatever reason & use a brand name tire shop. Most if not all will not sell your that one non authorized tire due to legal reasons thanks to Ford & Firestone for that. They will only sell you 4 correct tires for your vehicle. I ran in to that long ago

Would a 255 70 15 work?