if its a SU  its a 1.25"  or  1.5"  or  1.75"
(hint)1.25" carbs 850cc    +    1.5" 1000cc    +    1.75" 1275cc
but this only a guide

from memory.

early cars had HS carbs, easily seen by side mounted floaf chambers, either
1 1/4 or 1 1/2 inch choke size.
later cars had HIF carbs, with the float chamber built in to the bottom of
the carb body. either 1 1/2 or 1 3/4 inch chokes.  very rare you will find 1
3/4 HS carbs (cant remember them being fitted to any standard mini, but
often lifted from other BL engines) and i cant remember any cars with 1 1/4
inch HIF carbs.

some cars had twin HS1 1/4 and i think some "S" models had twin HS 1 1/2
carbs.

in the HIF range, the first ones were labelled HIF 6 and HIF 8 later ones
were labelled HIF 38 and HIF 44 as BL(rover group) tried to use metric
numbers... these are more or less the same.

I cant recall any mini based cars having twin HIF's as standard.

ones to avoid like the plague are the later HIF's with the auto choke
mecahnism.... I have had nothing but grief with them on anything other than
bog standard engines.

anyway
for the original poster, remember that if you change the filter to a free
flowing unit, you may risk weakening the effective mixture, good practise to
get the mixture re set afterwards

Jim