Do you have any kind of ruler?
Are you buying the new feet locally, or online? You could measure AND buy in one trip- look at websites for places that sells these and pick something.
I don't know where you are, but if you live in the US, Parts Express has a wide selection of spikes and feet for speaker stands, many come with threaded inserts that fit them. If yours have the same thread, great- if not, you can remove the existing inserts, install the new ones and you'll be done. You'll also have a set of threaded inserts for a future project.