The subject of the sentence is not the person Michael, nor is it the person Susan, nor is it even the combination of these two people. The subject of the sentence is a grammatical constituent and it consists of words. In this case, those words are "Either Michael or Susan".
Of course, we know that these words have meaning, but if you replace them with some other words that mean the same thing (like "Michael or Susan, not both"), you're no longer identifying the subject of this particular sentence. Instead, you're now talking about what the subject means.
This subject is grammatically singular. In general, "either A or B" is singular when both A and B are singular, and that's the case here. But as RegDwighт points out in his comment, it doesn't make any difference in this particular sentence. So, to test whether it's singular or plural, let's replace the predicate with something that lets us more easily tell the difference:
- Either Michael or Susan has a dollar.
- *Either Michael or Susan have a dollar.
Since have doesn't work, we can be reasonably certain that the subject is singular.