Some guys here
https://osu.ppy.sh/forum/t/106446
found a solution it seems. in the config file they refer to, you can specify region size.
Even though you might have the newest version, if the driver setup have not been modified too much, you should still have that aiptbl.ini config file lying in the driver folder which can be edited. You have to remove your current driver for the tablet, modify the file and install the driver. Repeat that action till you are satisfied with the active area. Normally when you get a tablet brand which is a bit more known, you tend to get such settings in the configuration program you receive with the driver, although it sounds like you do not have it. Maybe you could take a screen shot of the various tabs you have access to, then i can try to help?
also... If you should ever find need to get a new tablet, i can recommend Huion, they're cheap and the quality is really good, else get a Wacom tablet which is the State of the art of tablets but they are more expensive although the quality is also very good. I own a Huion tablet myself and it is everything i need, no issues and works on both PC and Mac.
Edit: Sorry i somehow missed your first two lines... so you say it does not keep the ratio? could you please take a screenshot and show me. You should be able to change the size of the active area as much as you like, that means it could be very wide or narrow, it should not depend on your ratio.
Take a look at my tablet configurations program for example, do have something that look similar?