WUJIAONIAOTravel Luggage Cover Baggage Suitcase Cover Protector Skins 18-32 Inch

Q: 21 inch suitcase what size to order?

I have a 22” hard side suitcase and ordered a size small It covered all surfaces and corners.

Q: Will it fit a 24-in. case hardshell american tourister burst max trio spinner luggage suitcase with the dimensions: 24"h x 18.3"w x 10"d?

You can choose from 4 sizes and would appear that Size M or L should fit for your suitcase.

Q: Does this product fit a horizontal suitcase with wheels? (medium glider)

Possibly. Think of this as a baby's onesie - there's an opening at the top for the telescoping handle. There's an opening on the side for the regular handle. The cover pulls down over the suitcase and has 2 large tabs that meet underneath between the wheels and zip or snap together. Even if your case is sideways, if the handle is on one end & the wheels on the other, and the size of the cover is appropriate to the size of your case, then I don't see why it wouldn't.