When a guild grows very large it changes. That's not necessarily a bad thing, but it does happen. Strangely it can be harder to find people willing to serve as officers, because it's more work managing a large guild. If your guild is still functioning well (and you must be, because you're not losing members), I think it is okay to limit your membership. I've heard of several groups that limited membership based on space, or even because they just didn't want to grow beyond a certain size. Also if you do move to a new location and accept new members you may lose some of the old members and wind up being the same size, but in a less desirable location.
I did belong to a very large guild in Oregon. It had a morning meeting and an evening meeting and two sets of officers, who met as one board. The morning meeting was very crowded in a huge room, and the evening meeting was sparsely attended. It seemed that only those who worked during the day preferred the evening meeting. Having the two meetings didn't really solve the space problem, but it did give those who couldn't attend during the day a chance to be guild members.