I know that this is an old topic, but one that has plagued me since switching to Windows 10, and I just solved it last night. So I'm answering this to possibly help other folks.
First, my situation. I manage about 16 servers from my home office. I have created saved RDP connections for each server to my desktop. Each day, I log onto each server using these connections to check logs, hardware status, etc. I do them all at once, so at least once a day, I have 16 RDP windows open at once. The constant downsizing, and snapping to the screen edge, was driving me insane!
Here is what I did to fix it...
By right-clicking each saved RDP connection, and selecting 'open with' and then choosing Notepad, you can get to a plain text file that defines the RDP connection (as opposed to the typical GUI RDP properties panel). On each of my saved connections, I changed the following 4 parameters: screen mode id, desktopwidth, desktopheight and winposstr! Here is what they look like on one of my typical saved RDP connections:
screen mode id:i:1
Screen mode id must be set to 1. That makes the remote session appear in a window.
Desktop width and height are self explanatory...but they can be anything so long as they're smaller than the screen you're viewing them on.
Winposstr is the important one. When you open your saved RDP connection file, the fifth and sixth values of this parameter will be much smaller in comparison to the third and fourth parameters than they are in my example. You have to edit those last two values...make them bigger! To get your RDP window to open full size, you have to add your desired vertical and horizontal resolution (plus a bit extra) to the third and fourth numbers respectively. So,
1575+1280=2855. If you put that number in for the fifth value, exactly, you'll still get scroll bars. So in this case on my system, I put 2900. Likewise, 160+1024=1184. 1200 still gave me scroll bars, so I tried 1230, and it worked.
I haven't taken the time yet to figure out exactly what the third and fourth numbers represent. It's obvious they're somehow defining the position of the window on my screen, but I just haven't figured out that exact relationship. And those numbers will probably be different for you...when you save your own RDP connection file, they'll probably vary because you have a different setup than I do. But the point is, you have to add the horizontal and vertical resolutions of your desired RDP window to the third and fourth number, whatever they are, PLUS a bit extra, and then replace the fifth and sixth number with the results you get!
Once you've made the change, save the file, and close it. Then when you double click that RDP connection, it should open full size, and be able to be dragged to whatever position you want, without automatically downsizing!!!!
On my system, the upper left corner of the RDP session defined by the parameters above will be 0 pixels down from the top of my screen, and 3 pixels from the left edge. You can change the initial position by editing these values...for example, 10,10 would put it 10 pixels down and ten from the left. But if you add to these values, you ALSO have to add to the 5th and 6th values as well. Like I said, I haven't figured out the exact relationship yet...just know that if you change these, you may have to increase the 5th and 6th values as well to avoid scroll bars.
Finally, on my system, I have the 'Prevent Windows from being Automatically Arranged when moved to the edge of the screen' parameter UNchecked (this is in the 'make mouse easier to use' section of the Ease of Access Center).