There isn't a
sizeof for specific objects in GDScript, to my knowledge. However, you can infer it by looking at the memory usage in the Debugger Monitors tab while running the game.
If your object is simple, you can also approximate it. For example, an
Variant size is about 32 bytes, so you can do
count * 32.
PoolIntVector, items are type 32-bit
int, so it would be
count * 4.
Another way might be to do the following:
var memory_before = OS.get_static_memory()
var obj = YourObject.new()
var memory_used = OS.get_static_memory() - memory_before
Even this might be a little off because of GDScript's execution itself in addition to your object, but that's a way to get an idea.
Someone could be tempted to use
var2bytes but this function actually encodes an object for later decoding rather than just dumping its exact memory.