For the OP, I would choose the biggest butt you can get or that will
fit the space available. If you can, get several butts and link them
together as near to the bottom as you can (butt suppliers do linking
kits). Linking them in that way means they all fill to the same level
and you only need one outlet tap.

In fact you can link together several butts that do not have low-level
linking kits, even butts that do not have ANY linking kits! You just link
them via their taps. I currently have three one hundred litre butts linked
this way. with more on the way. The way they make these small butts (I'm
talking about the "Space-Sava" type) means the linking cutouts are at the
top, for overflow use, not at the bottom as you get with the bigger sizes
which have a true linking capability. But linked via their taps with some
hose and some three-way adapters, and of course you have to leave the taps
open for this to work, they will all fill up equally with the rain only
flowing into one of them. This way you can add further small butts as the
need arises. As they are not deep from front to back they can line up against
a wall. Needless to say you have to turn all the taps to OFF before removing
this makeshift linking kit from and filling watering cans etc from one of the
taps. Or use inline taps as well.

The only downside I can see is that the tap flow rate is slow compared to an
overflow or linking kit, which seem to use 1" hose and have large apertures,
so in a really serious downpour the first butt might still fill and overflow
before filling its companion butts. The answer is to use the overflow
connections to link the butts as well so any overflow fills the other butts
before going anywhere else- then the last butt in the chain can have an
overflow that leads somewhere else.

It is surprising what combinations and configurations you can come up with if
you draw these things out on paper. Like for example an external water level
indicator that shows the level inside all the linked water butts. But it
seems I'm spending quite a lot on hose fittings.....

--
VX (remove alcohol for email)