If and when anything quantum is able to yield results (I wouldn’t worry much about this), increasing key size is pretty much meaningless, you need to move to other encryption schemes (there’s lots of options already).
Getting a working qc to reasonable scale is the hard part. Once you have done that most of the hard engineering problems are solved. I doubt doubling its size at that point would be very much of a problem.
If we are making (uninformed) predictions, i bet we wont see QC solving 1024 bit RSA in the next 15 years (at least), but once it does it will only take a year or two more to solve 4096.
It will likely work for a while, but it’s a fundamentally wrong approach and you’re going to be exposed into recording & decryption attacks, instead of breaking your encryption today, I just store all your communications and wait for the next qc to be online, then fish for stuff that is still useful.
It’s a silly approach if the timeframe is 50 years because most secret information goes stale quicker, but if you’re just waiting for say a year…