How do lawnmower blade sizes work?
Mower blades are generally sold with the length designation matching the mower deck width. A larger riding mower may have a swathe width of 42", but they'll usually use two or more blades in a side-by-side or staggered configuration rather than one large blade.

So before buying, figure out what brand your mower is, and what the deck width is. Then when you're looking in the store, match the brand and deck width with the mower blade set. The blades won't actually be 42" long - probably more along the lines of 21.5" or 22", since they have to overlap a bit for you to not end up with a center stripe of unmowed grass.

For example, this pair of blades meant for a 42" riding mower are 21.5" long each, and this pair of blades meant for a different brand of riding mower are 25.25" long each. The reason for the different lengths but the same overall width, is that the mowing decks have different geometries of how the blades are spaced and staggered.

The blades labelled 21" were probably for a push or walk behind mower with an actual deck width of 21".