Hi Shani,
wood is very porous and will definitely need coating.
Standard gelatine size is really for material like paper and probably not up to this job.
Have you tried PVA glue (the builders type - in 5 litre cans) which is aimed at sizing wood and plaster so it can be painted.
I dries transparent and is quite 'sticky' so should bond well with the cyanotype emulsion.

Good luck

Simon