Scrollsaw blades are usually 5-1/4" to 5-1/2" long. Ends may be plain or pinned: pinned ends are generally available on a smaller number of blades, because the pins force greater blade thickness, so smaller sizes are not made.
For most of us, wood-cutting blades are the standard, but metal-cutting scrollsaw blades are readily available. Most scrollsaw blades are skip-tooth design, one tooth skipped and a gap left, which allows for fast clearance of dust and chips. Spiral blades usually have coarse teeth (comparatively) and give a rough finish, but allow great ease in moving the work in any direction, with the corresponding cut going in that direction. Reverse tooth blades have some teeth (at the bottom of the blade) pointing up, rather than down, which reduces ragged cut edges.
The above is not an exhaustive treatise on types, but they are the most commonly used.
Bandsaws cut cleaner if the back of the blade is rounded with a small stone. Scrollsaw blades react positively to similar treatment, holding to the thought that the blades are much smaller and more fragile. Use a small stone, or piece of stone, in a finer grit, probably not coarser than 200. Gently round both edges of the back with the scroll saw on its lowest speed.
Universal Generic Numbering System (not all blades are categorized this way)
Please note that there is no stepping-stone sizing here, except in the numbers. Teeth per inch and blade thickness can stay the same from size-to-size, or vary considerably. And these are general sizes: manufacturers' sizes may vary a few hundredths of an inch in width, usually less so in thickness. There may also be variations caused by quality.