about the minimum population to be sieged , assuming you're lucky and gobs path to your fortress isn't bugged, look for the file named entity_default.txt located in your DF \raw\objects\ folder
Open it and for the dwarf entity , change the default
[PROGRESS_TRIGGER_POP_SIEGE:3]
to
[PROGRESS_TRIGGER_POP_SIEGE:1]
or
[PROGRESS_TRIGGER_POP_SIEGE:2]
to have gobs considering sieging you earlier.
from the wiki , the progress trigger value 1 corresponds to 20 dwarves, 2 to 50 dwarves, 3 to 80, 4 to 110, and 5 to 140
But you will very likely need to generate a new world, as raws modification usually require.
I am not sure if editing that progress trigger for the siege must be done for the other entities (human/gob/elf) that are all having the same value as the dwarves too :
[PROGRESS_TRIGGER_POP_SIEGE:3]
There are some other triggers , that can help siege, but by default they're disabled (set to value 0)
[PROGRESS_TRIGGER_PROD_SIEGE:0]
[PROGRESS_TRIGGER_TRADE_SIEGE:0]
I wonder if changing
[PROGRESS_TRIGGER_PROD_SIEGE:1]
[PROGRESS_TRIGGER_TRADE_SIEGE:1]
will actually make gobs more willing to siege if you reach early wealth/trade, or if it only make an additional condition to the population already.
Note that it does not guarantee sieges will actually happen, there are some problems in current DF that can make it so when you're not lucky , no gob will ever come.