I have never been told how big they are. In fact, I do not know, even after being shown them, although I think the answer is “not very”, as I cannot find any of them by myself, even after being shown.
Do not assume they will be glaringly obvious on your MRI. In my experience, it is hard for a layperson to interpret their own MRI results, and to distinguish between abnormalities, and ordinary structures of the brain. Also, because the MRI consists of 100s of “slices” you may have hundreds of frames that show nothing at all out of the ordinary, and just half-a-dozen that have something suspicious on them.
Unless your MRI copy comes already marked-up in some way with which frames are of special clinical interest (mine didn’t), you could pore through hundreds of frames without spotting a thing.
They do not need to be large to cause symptoms. That is why some people with MS experience symptoms even in advance of anything being visible on MRI. The presumed reason is that they do have damage which is too small to be picked up by the current generation of scanners.
More crucial than either size or number of lesions is location. It is possible that even a majority of lesions are “silent” - i.e. produce no symptoms at all. That is because the brain is very adaptable, and can often simply bypass or compensate for even quite surprising amounts of damage, so that no effect is noticed. But if you are unlucky enough to get a lesion somewhere critical, and the brain was unable to find an alternative route, or reallocate the work, that is when you start having noticeable symptoms.
Unfortunately particularly true of spinal cord lesions, because you haven’t got a viable diversion if that is obstructed, unlike some pathways in the brain.