I used to lay my tops generally around the suitcase and stick toiletries or things that might crack in between. Now that I use the cubes, the tops and underwear are in their own compartment. They do take up less space, because you shut the cube and it compresses it, if it's full. But now, I don't have anything to cushion my toiletries, so I have to be more careful and thoughtful about where I put things I would have just stuffed between two t-shirts.
Measure the inside of your suitcase, then look at the measurements of the cubes that interest you. How will you lay them out in the suitcase? Which will fit, stuffed (check the height). Also, take a sampling of the clothes you would pack in them, fold them and measure the size you need. Keep in mind that when you put sides around, 14.5 inches will squish into 14 inches. Or 13.78, as in some cases. Maybe try putting them in a plastic shopping bag to see how it would work for you. Then, buy the sizes you need.
Not all packing bags are the same sizes. If no size is listed, pass. If there aren't 2 zippers, I'd also say pass, although that's not a deal-breaker.