A 12ltr cylinder is big and heavy, difficult to move about if you want to take it with you to a shoot. A 7ltr (which I have) is still fairly hefty and the largest I'd contemplate as being 'conveniently portable'. If you can afford it... it's nice to have a 12ltr at home and a 3ltr to tote about with you, you can top up the smaller one from the larger one with a decanting hose. By all means get a 300BAR cylinder, you have a regulator on the cylinder so you only top your gun up to the pressure required for it. If you get a 232BAR cylinder then as soon as the pressure in it drops below 200BAR you can only partially refill your gun (if 200 is what it needs...). You can never use all the air in your cylinder, once the pressure starts to drop you'll have to refill it, and a 300BAR cylinder will need filling less often.
I got a full 7ltr kit from here: http://www.bestfittings.co.uk/shopdi...=7+Ltr+300+BAR
HW100KT, ATN X-Sght 4K Pro, MTC Taipan 6-24x56; HW97K, MTC Viper 10x44
Walther LGV Comp Pro, MTC Connect 3-12
HW45, Beretta 92fs, Premier, Tempest, MkIII, Orig. Mod50