Normally there are 8 sizes ranges for luggage set.
2pcs Luggage Set: 20/24inch, 24/28inch
3pcs Luggage Set: 20/24/28 inch, 18/22/26, 20/28/32inch
4pcs Luggage Set: 20/24/28/32 inch, 16/20/24/28inch, 18/22/26/30inch
Of course, the size ranges for luggage set can be customized, or as per cutomers' preference.
For current customers, below size ranges are common.
3pcs Luggage Set: 20/24/28 inch
4pcs Luggage Set: 20/24/28/32 inch
for both soft and hard case luggage Luggage Set.
And for hard type Luggage Set, 20/24 size range is very common, as size 28 is used not very often.