Fan of these
https://noaks.de/Seem pretty durable and the edge is welded 7mm wide, so hasn't blown out at the edge like a regular zip lock would over time.
Though if I had to guess the S size is smaller than the PSP, and the M larger.
XS size fits a 2 Maxi Bic lighters pretty much.
Here is a pic from last year of my spare fire kit in an S.
SAK Walker
Lighter
LMF Army Rod
Tin (80mm x 50mm x 15mm) with dozen tinder quiks, and a dozen storm matches.
Two tea lights
Since added an expandable bellows, replaced the Army rod with a Scout, and the tea lights have been replaced with 4 7g dragon fuel pods.