It’s typically in relation to the piece in question. This is often used with the technic beams so it’ll say a size 3 with a pic that has 1:1. This means if you pay the piece down on the image it should match exactly so you know you’re using the proper piece. Fairly pointless in the sets with a single size but a friggin god send for those sets that have 30 different sizes.

If you’re wondering about the numbering scheme, grab the longest 1xN piece you’ve got (not required but they’re often easier to handle, anything else is acceptable such as baseplates, lol), and lay it down flat and it tells you how long it is in the number of studs.

So a size 3 beam is 3 studs long. :)