I understand your concern. I like to shop during my trips. I also like to stock up on certain supplies, much of which is heavy. Or getting free bottles of wine as gifts, as another example.

As far as size is concerned. for 1-2 weeks I use a 26" rolling suitcase. For 2-4 weeks I have a 28". And this takes into account packing for various climates, hot and cold, different occasions, suits and hiking or multiple shoes, dress, casual and athletic. And bringing back shopping and gifts.

a 20"-24" bag is too small for your purposes.

I agree with Trooper's advice. Bring a second large flat soft bag in your main large luggage and use it for the return trip. I never travel without a spare bag folded flat in the bottom of my luggage.

And spinjockey also has good advice regarding a scale. I always have a scale packed in my luggage too. the 28" can hit 50lbs very quickly, sometimes only at half capacity. And if I pack the 26" very tight it will easily top 50".