This is some real “Flight of the Navigator” type stuff going on here.
The relationship isn’t linear so it wouldn’t work exactly like that (double the size of the planet and time slows by half). The speed of light example I gave above is closer to a linear relationship if you imagine velocity on one end of a slide rule and time on the other. We spend our entire lives on the far side of the ruler towards “time.” Someone like Mobius or S2000 would have to actually do the math, but if you were traveling at 50% the speed of light, time would slow down by about 50% relative to those back on earth.
You would be living in slow motion relative to him (dude on earth). Depending on the gravity and/or speeds you’re traveling through/at, you could come back to earth and find everyone you knew long dead and your great-great grandson is now an adult. But, you and the people of earth have experience every second or moment. It’s just they’ve had many more of them.
You’re overthinking the problem (which is very common since it’s a hard one to conceptualize). A second for you, the space traveler, will always be just a second. You’ll never feel any different. Same goes for the stationary earthling. We’ve actually done this experiment with pilots. Here’s an article about it.
For the next step, read up on what happens to a space dude and an observer when the astroperson falls into a black hole (or, more accurately, passes the Event Horizon). Spoiler - To the person passing the event horizon, nothing appears to change. They just keep falling until they die an extremely brutal death. But, to the observer a safe distance away, the spaceman never crosses the event horizon. Time stops for the spaceman while the observer keeps watching from a telescope waiting to see space buddy keep falling and get spegettified.