It adds about another 1cm to the depth of the mod
@MarshallMoore, so it's large but not uncomfortable to use.
And there's a rubber bung on the back of the mod that you remove, then you attach the Squonk backpack and finally secure it with 2 screws. Just decently tight is enough.
And you can use any RBA or Tank up to 36mm diameter, without any overhang on the Hadron's as well. But you don't need to if that's not your thing.
It is a pump type Squonk though, which means that every full press of the pump button despenses 0.4 ml of juice or liquid. So you have to be mindful to not pump too much juice or liquid, or it gets messy fast.
But Squonk mods allow you to use RDA's without needing to drip liquid or juice into them, so it makes life so much easier.
But RDA's and RDTA's are probably the easiest RBA's to work with, and they are a bit of a jump when it comes to flavour and vapour. A good basic example is Cuthlhu Mods Mjölnir RDA, I highly recommend them for ease of build and flavour/vapour.