Wood rafter

Im looking to build a leanto on side pole barn 25x40. Want to use my own timber cant find answer to what size solid wood beam i need to span no center post

What you need is a rafter span table, if by beam you mean the rafters, and you are using dimensional lumber. Not sure if you mean a span of 25'; if so that may be pretty substantial, esp. If snow load is a thing where you are...

If you mean the beam supporting the outer end of the rafters, you'll not be getting a 40' unsupported span.. and will need to do some math. Rob Roy's 'Timber Framing for the Rest of Us' has some good content for this, but I am sure there are plenty of other options.

If you have not built before, keeping things at say 10-13 foot spn will make the job easier and siutable logs will be easier to find.
Also you may get away with a tripod to left them rather than a crane

Less than 12' is usual around here with a 2x10. But a few years ago I built a roof that spanned 40' we used two (a double) LVL beam 24" tall.