Ceiling Boards

The garage being converted is, as I've previously mentioned, 4900mm x 2480mm - i.e. just bigger than would be ideal for the 2400 x 1200 sizes of everything.
As my walling+insulation will be over 50mm on each side, is it OK to just have one piece of 2400 x 1200 spanning across the garage - i.e. 40mm gap either side, which will be hidden by wall, and which will be covered by insulation from above? In this way, I could cover the entire garage using 4 or 5 pieces of plasterboard.
Also - when doing the drylining technique - what is a typical distance from the wall for the boards? I know it will have to be big enough to hide cables and pipes - but are we talking 10mm, 15mm?
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