Make it in a vector program such as Adobe Illustrator, then it is scalable ie, you can blow it up and stick it on the back of a bus and the image does not degrade.
Try not to have any raster content (photos etc) or it just defeats the purpose of it being a vector.
Try not to use too many gradient fills and watch the live paint, sometimes it is not very editable.
Start with it approx A4 - 210 x 297mm and when you have finished it, copy and scale some smaller for letterhead, membership cards etc to see how they work and check the legibility of the font. Speaking of fonts, try not to use a too way out one that no-one else has, or if it is for the logo only, make sure you create outlines and not leave the font intact.
Now, have I missed anything?