Rule of Thumb is, the smaller the seed, the longer the viability. Also seeds with a hard shell last longer. For example, The Coconut. I read somewhere that they germinated one that was estimated to be 200 years old!
So, stockpile coconuts, LOL!
Also, storage is of utmost importance. I’ll ping the Seed Saving Expert to see if she wants to post a short primer. ;)
I haven’t found any correlation between seed size and longevity. There is a definite correlation between the hardness of the seed coat and longevity, but even then there are exceptions. The biggest two factors are humidity and temperature. According to the guy who runs the Baker Creek seed warehouse (awesome guy), the goal is to keep the temperature (in F) and the percent humidity low enough that, added together, they equal less than 100.
In 2020 I took a bunch of old tomato and pepper seeds, some as old as 1997, and planted them. Most were for varieties I didn’t really want, but they came free with a catalog order. I assumed most wouldn’t germinate, especially since they’d been stored near a heating vent in the same room as the humidifier, so I was mainly just getting rid of old seed.
Every single one of them grew!