What would you do? 24’ or 28’?

My advice would be to get the bigger one now. Look for something in a bunkhouse model. These usually have a queen bed in front and bunks in the rear. You will, however, have a hard time staying under 5000# total, but your truck can pull more than that no problem. We just ordered a Grey Wolf 26BH (by Forest River) and I believe the dry weight of it was 4200# and it's a pretty basic TT. It doesn't take long to start adding weight to them either with clothes, dishes, pots, pans, etc. Slide-outs are nice too but add more weight and are another place to leak, also. Our last TT had a small slide that added some nice space inside, but our daughter is getting bigger and wanted her own bed instead of the fold down couch. Our new one does not have a slide, but that's ok, we don't spend much time indoors unless the weather is bad.
As far as the length limit, I'd guess that is the length of the camper. It'd be hard to stay under 30' with any decent sized camper and a pick up truck. I think my DoubleCab Tundra is over 20' long, or close to it from bumper to bumper.