Typical roof joist spacing, as boo1 suggests, is 24" o.c. Other typical spacing is 16". Standard sheet of plywood is 4'-0" x 8'-0".
To determine a reasonable spacing for the wood trusses, first select the typical joist you would like to use (say 2x4 or 2x6), determine a maximum span for the load condition, then use a even number feet that is less than the maximum span.