what is the smallest size to put male guppy in?

I have kept 10 guppies in a 5 gallon tank with a 20 gallon filter before i moved them to my 150. They were in this tank for 2 months and they did fine. You can throw a male guppie in a .5 Gallon tank.

Guppies are docile fish and they dont fight.... they will be fine.

A 10 gallon would be the smallest tank size for a male to permanently be housed in. You could also get a divider for whichever tank you have now and keep the males on one side and the girls on the other. Male Guppies don't mean to bully, they are just constantly trying to mate with females. If you have at least 2 females per male the girls can get some rest, although you won't naturally get that ratio when they have fry so you might have to rehome some anyway.

If it's just one guppy, then maybe a 5 gallon would be the smallest. But a 10 would be better if you could.

He's not bulling her. He's probably mating with her. What do you have them in and is there any plants or decorations? He's not going to kill here. They don't do that. Just leave them alone and let them get use their new home. Get them on a feeding schedule and just let them be for now.