Just remember to take a tape measure with you, especially if you are planning to buy a larger rolling suitcase or duffel. The dimensions listed on the label are often inaccurate, and you wouldn't want to depend on that and end up paying an oversize fee when you check the bag.
If length + width + height (included wheels and handle) add up to more than 61" on a softside, don't buy it.
I'm picky about luggage, and I usually buy it at either Tuesday Morning or TJMaxx. One thing that I always do before buying is to roll it over to the linens dept and fill it to the brim with packaged sheets, then zip it up and re-measure it, and also roll it around a bit. If it is awkward to maneuver when fully packed, it's not for me. (It should go without saying that when I've decided what to buy, the sheets go back exactly where I found them.)
Also, re: warranties and luggage. If you plan to ever invoke the warranty, save the labels AND the store receipt for the bag purchase in a safe place. Most luggage makers will not honor a warranty claim without them, even years later. I found that out the hard way.