The monitor has 3 levels, low, high and peak fertility. Usually I would get a few days of "high" before peak... a few times I actually got about 67 days of "high" before peak and I actually thought it wasn't working. But there has been one time that it went from "low" to "peak" and a few times I only got one day of "high" before "peak". The high detects the rise in E2 and the peak the rise in LH.
It is definitely compact. I'd say about 5 inches by 3.5 inches and about an inch thick.