First of all, welcome to Babypips. I will try to answer your question to the best of my ability, more experienced traders I am sure will chime in and offer their advice too.
The 1:2 risk:reward ratio (RR) is a general guideline as to how to use good money management skills. It shows in a statistical way, if all things remained equal in a controlled environment (meaning if you loose a trade, you loose everything you risked and if you won, you won exactly twice the amount you risked) how you can win 100 trades and loose 100 trades with a 1:2 RR and still come out ahead. However, that is simply an example to show you how a 1:2 RR statistical edge works. In the real trading environment, things are very dynamic and nothing is constant. 1:2 RR is a reference point when examining a chart and determining if it is worth the risk of opening a trade.
Once a trade is opened, it literally can go anywhere. You may end up with 1:-1 or 1:10, however, 1:2 RR is a good rule of thumb to gauge an entry. If you have a strategy that allows you a clear picture of of entry and exits, then by all means, plan for a bigger reward if there exists one. The key word here is strategy. R:R will only work when a trader HAS a solid strategy that has an edge, meaning that statistics and probability will lean in the favour of his or her strategy over a given period of time.
As far as your question about sizes of trades affecting the 1:2 RR, yes, you are right, if a trader changes the amounts of R:R from one trade to the next, and if they have no idea of what they are doing, they will burn their account regardless of what statistical probabilities are at work. Haphazard changing of amounts from one trade to the next indicates emotional trading which is essentially gambling.
The general logical progression for a trader is to start small and steadily work their way up from there. When you have a solid strategy, a sound mind and some experience, then when you apply 1:2 RR, it will be profitable in the long run.
I hope I have helped answer your questions. Welcome again, thanks for posting and best of luck in your journey!