Bicycle trailer wheel size
My expectation with this design is that you'll be able to carry 15-25kg over a fairly rough track, if you built it reasonably well. Use the heaviest bike you can find as the donor, you want nice thick tubes. But it will probably fail by bending close to one of the joints. Bike tubing is not really designed to deal with bending.

With the design sketched I think that'll work fine with 26" wheels. The main thing will be welding the steering on the trailer frame/donor bike so it doesn't wiggle. Because the fork/wheel is going backwards the rake on the steering will be backwards, so if it's loose it'll be like trying to roll a castor backwards - very wiggly and unstable. Turning the fork round will help, but I suspect not much.

The suspension will probably not work as well as you hope because a fair bit of the force will be across the line of the suspension movement - rather than "brake dive" you'll have... nothing. I'd be tempted to try to get the fork more vertical, but that could end up making the design more complex and you're better off making something quickly, riding with it, then fixing the things that annoy you most once you have something that works.

Chainstays, your "bottom part", are really not designed to take bending loads, so that will be your weak point (assuming your welds hold). I would be tempted to brace those somehow, even just putting another tube between them with welded joints at the ends to reinforce that area will make the trailer stronger. If you have a space bit of tubing that would be well worth while. Gussets, BTW, will not help (here, or almost anywhere), they just shift the point of failure to the end of the gusset - designing an effective gusset is hard.

That ExtraWheel trailer design is all about a triangulated frame, and you could probably fake it using the seatstays and chainstays off two bikes, welded to a headset. It would probably be easier to build, but the attachment to your bike would probably be more tricky. I'd be tempted to bolt into the rack eyelets, if you have them, and see what happens.