You should look more into what can be installed on each distro, since most are built on a similar base (eg ubuntu, debian, arch). I would recommend looking into something based on ubuntu or debian personally, because they're the most common and therefore it's easier to find software packages for them.

Basically, with Linux, you can customise the UI and font sizes as much as you want. You don't have to worry about that stuff like you do with Windows or Mac. If you find that a program won't allow you the customisation that you want, then you can just uninstall it and replace it with something else. Even if a distro doesn't seem accessible at first or is missing something that you need, you can just replace it or add what you need. I should note that large amounts of customisation can be a hassle, so while it does give you the most freedom to make your system as accessible as possible, don't aim to just build your whole system from scratch if you're not familiar with Linux.

When I use Linux, it's with a screen reader so I am not sure. Just don't use any distro with cinnamon, because the guy ripped all the universal access stuff out. I tend to use Arch mostly.

I used to use Ubuntu with XFCE since it was pretty lightweight and configurable.

If you're lookin for the easiest way to get started and not install anything or set up much, look into vinux. It's a team of people who set up a lot of this already and put it in a package. Even braille can be used during setup. Good luck!

I've messed around with Ubuntu. Unity tweaker can be installed, and you can change the system don't with it. Ocra screen and magnifier come preinstalled with my version of ubuntu

But I expect you are able to change anything you want, since it's Linux.