What would be the rcommended fret size for a ES125 guitar from the 50's

If that were my guitar, I'd refret it using the wire size I like.

The problem is there was never any 'standard' for Gibson fret wire; it depends on what they had on hand. Some guitars came off the line with mandolin wire; this is what led to the "fretless wonder" label assigned to many guitars in the 50s and 60s, and resulted in Gibson sticking with that size for certain models of Les Paul in the 60s. Gibson also used what today is called medium jumbo wire, so that is appropriate too.

Case in point: I have a 1960 Guild that sounds fantastic but had the tiniest frets, so I refretted it with jumbos because that's the size I like.  I'm glad I did.

Cheers, mate.

Make sure to compare neck sizes too, because that affects how frets feel. For example, a thinner neck with tall frets can feel like a thicker neck with small frets. You can use a dial or digital caliper to measure the neck thickness and width, as well as the fret wire.

For players that are unfamiliar with how to use a caliper, it's pretty simple.

Here's how to do it using a dial caliper, which I prefer because there are no batteries to go dead.  If you have a electronic digital caliper, it works the same way.  Take measurements from five or six different places on the fretboard because height can vary if the frets have been leveled and polished.