To be honest I don't think it matters too much what size cylinder you use, but if you're eventually looking at kegging in Cornies then I'd personally suggest that a "full size" (6.35kg) one is going to make the most sense over the long term.
The 6.35kg cylinders are about 850mm tall which is unlikely to fit
inside your fridge for pressure fermenting, but that needn't be a problem because it's dead easy to feed the gas in through the drain hole in 3/16" tubing. To be honest it's probably more convenient and flexible to have the cylinder outside the fridge anyhow
(important safety note: make sure you chain it to something solid to stop it falling over - a smashed regulator is not a good thing to experience).
If you don't fancy a 6.35kg cylinder then the other sizes typically available are a 'half size' (3.15kg / 500mm) or 'quarter size' (1.5kg / 450mm) but, as you'd expect, these come at a bit of a premium.
The Adams Gas website has some decent info on sizes:
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