This is a strange one. I open a new assembly and insert a single part on the origin. The planes are whatever size there are from the new assembly. I RMB click one of the assembly's planes expecting to see 'Autosize' in the menu but it's not there. If I manually resize one of the planes, that plane's RMB menu now shows 'Autosize' but the other two don't. I must do at least a minor manual resize before I can autosize. Is there some good reason for this? I don't remember previous versions acting this way.