Sometimes the cursor size gets out of sync with the current scale of a Microsoft Remote Desktop virtual screen on the macOS app Version 10.7.6 (1972). So the cursor might be gigantic or really tiny...
I'm also experiencing this pretty most days. Some additional info I've discovered in case it's helpful:
Sometimes if I close the window and reconnect the mouse size will no longer be an issue.
I'd have to test this some more, but it might only be happening to me when I have my MacBook Pro connected to another display.