Anyone know what the ideal overbore size is for my 78 TY250?

It depends on which bore you are on now.

and how worn it is on that bore

Can't vote without measuring the barrel ...!

One that you can get a piston to fit ?

Still has original piston. I had heard that going 60 over is about it. Just wondered what others have found that works best.

If you want to stay around 250cc, then get the bore measured by the person who will be reboring it. They will tell you from that what size oversize piston to buy. You should always get the smallest oversize that will clean up the bore, so you then have more options for the next rebore.

If you want a bigger displacement, you can get pistons up to 2.0mm (0.080") above standard diameter off the shelf made by Wossner, which increases the capacity.

TY250 motors can also be made 320cc but this is a lot more expensive because there is a new sleeve and more machining required, and rejetting, and a heavier flywheel

Thanks