I'd imagine you'd be able to put in whatever size SSD you want to since your existing HDD runs through SATA. An M.2 drive will provide you more throughput than an SSD since it's using the PCIE bus instead of SATA6 (which I assume your board has given the other specs). From what I have found....
You will need to get an M.2 2280 chip. Anything other than a 2280 will not work because you dont have the mounts or size for it.
You can clone windows to the M.2 chip but there's not much of a point. Leave windows on your HDD, store all your critical documents and install programs onto the M.2. You'll get the M.2 speed when using those programs with the large capacity of Windows on the HDD. If you're insistent on getting Windows onto your M.2 device, you'll need to download the windows 10 ISO to a flash drive and do a clean install onto the M.2 chip. Microsoft has a how to on that here: https://www.microsoft.com/en-us/software-download/windows10
The storage size on the M.2 chip shouldnt matter. As long as you have the 2280 form factor you are fine. This video shows installation of an M.2 on your computer at 500GB