I think we could get away with leaning on another agency’s work toward defining bag sizes to help us out here: the TSA has a maximum size for carry on bags set at 22" x 14" x 9", including handles and wheels. That’s a relatively small bag size, but I don’t think there is a standard maximum in each direction like that for checked bags. Those bags can be bigger, but the restriction is in 62 linear inches (i.e. the total sum you get from adding together the width, length, and height of your bag has to be less than 62")–not a maximum in each dimension.
People tend to be familiar with checked bag sizes. There is a max in each direction already established by a well known industry/agency. And since it’s a smaller bag, you can more efficiently cram the nooks and crannies of a vehicle’s cargo area with them (the same can’t be said for the larger, 24", bags that Alex on Autos uses–I like his general method here but there is no true standard for a 24" bag like there is for a “checked” / 22" bag).
the problem is luggage size varies so widely it’s just impossible to standardize that sort of thing in any meaningful way.
I think we could get away with leaning on another agency’s work toward defining bag sizes to help us out here: the TSA has a maximum size for carry on bags set at 22" x 14" x 9", including handles and wheels. That’s a relatively small bag size, but I don’t think there is a standard maximum in each direction like that for checked bags. Those bags can be bigger, but the restriction is in 62 linear inches (i.e. the total sum you get from adding together the width, length, and height of your bag has to be less than 62")–not a maximum in each dimension.
People tend to be familiar with checked bag sizes. There is a max in each direction already established by a well known industry/agency. And since it’s a smaller bag, you can more efficiently cram the nooks and crannies of a vehicle’s cargo area with them (the same can’t be said for the larger, 24", bags that Alex on Autos uses–I like his general method here but there is no true standard for a 24" bag like there is for a “checked” / 22" bag).
Just use the max size for luggages you can bring with you on flights. It’s standardized and readily available in any luggage store.