Short answer: You can't. You're playing an out-dated game on a modern computer designed for, and capable of so much more.
Long answer:
When RS started some 15 years ago computer screens were small and had lower resolution so the client size needed to be small for everyone to be able to run it. Jagex didn't bother increasing the client's size as display technology improved (remember the infamous black area around their webclient?) likely due to Java limitations at the time, until the high definition version was released in 2008. Roatpkz is a 317 client which is around 10 years old - therefore it pre-dates the official resizeable release.
Technically it is possible to add resizeable as a custom feature to a 317, Gretar or Zoradz might know how. It's been suggested before.
The best you can do right now on your end, is change your screen resolution to something retarded like 800x600 (like old CRT monitors) which just stretches the client rather than offer true scalable full-screen. But I suppose it looks better visually.
The thing is, it wasn't always this small, i remember a time ego where it was like 50% bigger
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Client size problem
in General Support
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The thing is, it wasn't always this small, i remember a time ego where it was like 50% bigger