50 lbs is really going to depend on what type of stuff you're bringing back. I typically run into the weight limit before size becomes an issue.

For example, if you're packing books/bottles/jars you'll hit 50 lbs in a 22" rollerboard pretty easily. Clothes only I'd guess a 24"-26" roller board to hit 50 lbs (I don't own anything larger than a carryon, going based off friends). Packaged boxes (e.g. wrapped gifts) you'll probably hit the size limits before the weight.

I'd recommend taking a backpack for a carryon with extra change of clothes and anything too valuable to check and a ~26" rollerboard that gets checked. When/if you need more space then get a sturdy cardboard box or buy some cheap luggage along the way.

Most importantly I'd bring a luggage scale and stay a pound or two under. I'm guessing you'll end up with some heavy stuff in the carryon.