Cold water storage tanks - size matters??

Morning folks,
About to start the prep work for putting a new cold water storage tank in the roof (old one is too low and gives pants hot water pressure, hence the need to raise). Because the old water tank is getting on a bit, and because of the timescales involved, I'm buying a new one (going to install the new kit and do the pipework as far as I can one weekend, then disconnect the old / connect the enw the next weekend), and was wondering about sizes - will the size I choose matter much? The tank only feeds the hot water tank, and a 20 - 25 gallon (ish) one seems more than adequate - is there any real advantage gained in having a bigger one?

Normal for a house is bigger, like 50 galls. You have to make sure the thing will fill fast enough so that when you run a bath you can't empty it. If you decide you want a decent shower and need a cold feed, that would come from the same tank, by the way. Easier to stick a tap on it now. Use Full Flow Ball Valves, and a decent make, such as Peglers from screwfix.

http://www.screwfix.com/sfd/i/cat/85/p1941585_l.jpg

You can get "coffin" tanks which are easier to get high. Leave a space over it to allow access to the float valve

Cheers fella,
To be honest if this doesn't up the H/W pressure enough for the existing shower head (which is just a one from the mixer tap) then option 2 is to convert a small storage room downstairs into a shower cubicle. Assuming hot water only from the tank and no cold would a 25 gal do?? To be honest I'm probably gonna get a slightly bigger one - aim for 50 Gal - time to check out screwfix / local plumbcentres

Ta for the advice