The name is about lifespan, not size
A methuselah starts small and takes a long time to settle. There is no single universal cutoff, but patterns that stabilize quickly are not usually treated as methuselahs.
The R-pentomino is the classic example: five cells, a 3 x 3 bounding box, and a stabilization time of 1103 generations. That mismatch between small beginning and long history is the point.
Long-lived does not always mean surviving
Diehard is useful because it stays active for 130 generations and then disappears completely. It teaches a different lesson from the R-pentomino: visible activity is not the same as lasting ash.
Acorn pushes the other direction. Its seven cells produce a long, messy evolution before the reaction resolves. It is a better pattern when the goal is to watch a population curve rise, collapse, and rise again.
The lab needs enough room
Methuselahs punish cramped boards. If the boundary cuts off debris or escaping gliders, the observed result is no longer the natural evolution of the seed.
That is why the simulator includes larger board sizes. For R-pentomino and acorn, a roomy grid is not a cosmetic setting. It changes whether the demonstration is honest.
Working takeaway
Use single-step mode at the beginning, then speed up after the first expansion. The early generations explain why a tiny seed can become a field of debris.