The larger you go you get two drawbacks:-
- The bigger the wheel the more space it takes when turned. You get either less turning circle or you need to make the trike wider to allow for this or a combination of both.
- The bigger you go the bigger the effect that lateral forces have on that wheel. Bear in mind that bike wheels are not designed to take any lateral forces at all. You can get 48 hole rims in 20" but 24" don't tend to go above 36 holes so you're weakening the wheel further. BMX 20" rims also tend to be made to take a bit of abuse. I've used 20" 36 hole rims on tadpoles and they've been fine so far. I've seen many people say they've used 26" or even 700c wheels without incident and others say they've "taccoed" the bigger wheels without trying too hard and 20" is the max to use.
20" wheels never come with 20mm hubs or even 15mm. 14mm axles are fairly common and are butch enough but getting them with disc mounts is like asking for rocking horse droppings. They do exist but are super rare. If you are to find 24" wheels with a thru disc hub I'd start the search on ebay but given 24" is a bit of an in-between size I'd not expect to find anything. Lacing your own isn't exactly a difficult task, just a little time consuming and of course often more expensive than being able to find them in the first place.
Personally I'd stick to 20", especially so if you intend to give it any speed in a bend, but if you choose to go larger keep the spoke count up, choose a double walled rim, and don't forget to alter the chassis geometry to accept the taller wheel.