Measure distance fr dropout to brake pads. I also see that your calipers have a good deal of adjustment for brake pads. Just by looking, maybe a 16" 349mm wheel or an 18" wheel.
The diameter of a 349mm 16" rim is 360mm. So if the distance fr dropout to brakepads is below 180mm, (maybe 175mm to account for rim brake surface width), then it should be a 349mm. I've never built an 18" wheel so dont have rim measurements.
Hope this helps