Hi, I'm currently trying to create a fishing game and was having trouble determining a scale in which to make the fish and player. For example, would it look out of place if a goldfish were bigger than the boat trying to catch it? Or should I make the goldfish smaller than the boat?

The problem I'm having with scale is that the bigger the fish are, the bigger the world, which could lead to optimization problems. I also don't want to make the fish too small as that would make it harder to see. (As an aside question: If I'm working to make a game for PC is there a size I should stay above so it's not too small for the player to notice?)

Does it make a game look better if it's too scale (or close) or does it not matter?

I think it depends on the style and approach of the game (which you can decide :))
Do you want it to be realistic, then scale everything like/ similar to reality. Or if you go for fun, play a bit and "highlight" things with bigger scale as you like.

Taking Newtonian Physics into consideration, we would use 1 (computer simulation) unit == 1 (real world) meter

It depends on the style of the game as @cozzie said. I would go for something more realistic.

Some important notes you may take into consideration:

If you scale a object this will not only contribute to its visibility size but also you need a way of make sure you're collision geometry it is up to it.

Changing scale you'll need to change the way you compute the normal matrix.

The scale it is just an superficial thing that is usefull for model reusability. So if you change the size of the fish actually you're changing the size of the fish, and not decreasing the size of the world. Actually this makes more sense than vice-versa since there is no real-scale on objects on real life, it is just an power button that was pressed when you're in god mode (or on the model workshop). Note that I'm not saying mathematically because this is another thing related to linear transformations, composition of linear transformations, etc.

There are two matrices: the model matrix and the rigid body matrix. There is no scale in the rigid body matrix, only its 6 DoF that is converted to a rotation matrix and a position. The model matrix can be whatever composition of linear transformation you want (just make sure it is orthogonal ).

There are some engines out there that does not use scale in the engine at all! (saw that here once, don't remember when or where).

So, making your models "normalized" as possible could be a good solution.

This has not much to do with the question, but I'm reminded of playing "Black Bass" on the NES when I was young. I had borrowed it from a friend, had no instructions, and had never seen it played. I played for awhile with some small silhouettes of fish swimming beneath my lure, but they would never, ever strike at it. After probably hours, I happened upon a larger fish -- at least 4x as big as the silhouettes, and that fish on occasion would strike but I never could get him hooked. More hours pass, concentrating on trying to catch this one big fish -- and finally I got him hooked! I mashed the button as quickly as I could to reel him into the boat, excited to see how big my catch was...

And then as the fish reached the edge of the boat, and ENORMOUS hand reached down and plucked the tiny minnow I had just spent hours catching from the water.

And that, kids, is why I never played Black Bass again.

There is a lesson to be learned about scale here, however, and that is that scale means nothing absent a well-known reference. Is that fish under the serene blue surface of the lake tiny or enormous? NOT A CLUE! Its bigger than the other fish, what now? STILL NO CLUE! MAYBE THE OTHER FISH ARE JUST SUPER TINY! Look, its at least as big as that lilly-pad, its gotta be big, right? HELL NO! SURE, THEY WANT YOU TO THINK ITS A NORMAL-SIZED LILLY PAD, BUT MAYBE ITS TINY TOO! OR ENORMOUS! NO ONE KNOWS BECAUSE THERE'S NO HANDS, COKE CANS, OR BANANAS IN THE FRAME!

I think it depends on the style and approach of the game (which you can decide )
Do you want it to be realistic, then scale everything like/ similar to reality. Or if you go for fun, play a bit and "highlight" things with bigger scale as you like.

If it was more cartoony or pixelated style, would it look weird if a bass was the size of a boat? Or could it possibly make sense that underwater the fish look bigger and then once they are reeled up they become smaller?

Why not introduce real world sizes closity, and involve some gold-fish-hunter perception scale up if goldfish is far away and scaled down due to perspective look.... you might apply it only on few of them (closest? targeted?)