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.