I'd say you would be fine with a 60x90 pen. If it is only going to be your horse out there working, that will be plenty of room for figure eights, most patterns, really anything you need to do for a western horse. We show all year round, and ride in a 60x90 arena during the winter. It can get a little cramped with multiple horses, but for one, you should be fine with that size.
I have a 50'round pen and my arena, well OK the roped off bit of pasture that I school in, is 66'x 198' which is 20m x 60m the size of a dressage arena, suits me fine and I am the only rider here, so no sharing needed.