if you're trying to estimate the size of a given datastructure, there is a feature request for it, but in the meantime

v8.serialize(foo).length

in a node shell is reasonable estimate at least for storage size. If you're trying to estimate how much memory a whole app uses, local testing with minflare is probably your best bet. You can tell whether instances are sharing memory in production by just checking / setting a global variable in the constructor (if it was already set, you're sharing). There is no maximum number of instances that can share memory