Wow, 20x24...

After years of making small prints (nothing larger than 8x10), I set up to do 11x14. I did it because I had an image that screamed at me that it needed to be larger. It was right, it did. The image always knows best what size it needs to be.

Even going from 11x14 to 16x20, I think you will notice the difference in the quality of your prints.

Try this. Raise your enlarger head to the height it needs to be to make a 16x20, then print an image (one you think looks really good at 11x14) on four sheets of 8x10 paper. Process, dry, and put together to make a sorta-16x20. Do you like it more or less than the 11x14?

I wouldn't go up in size unless I had images screaming at me that they need to be larger.

Technically, you'd be better off with a 100/105 than your 75mm lens. The longer the lens, the more of the negative that's covered by the middle of the lens, rather than the edges. The flip side of that coin is, longer lenses require more distance between the lens and the paper to make the same size print. At some point, you can't raise the enlarger head high enough to make the size of print you want. But you would be fine with a 105.