The current default value is 1GB (1024**3).

Given that the default block size (a consensus parameter, not a config one) was reduced to 4MB, we should adapt the default maximum size in bytes for the mempool accordingly.

For instance, with maximum-size blocks, the mempool would be drained after 256 blocks by default. A factor of around 10 would be advisable.

Originally posted by @sergio-mena in #2352 (comment)