Is there a way to get mouse cursors bigger than 64x64?

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It looks like you've already exhausted the cursor options in the GUI system settings accessibility menu in GNOME.

To set a custom cursor size using the terminal, you can use the following command:

gsettings set org.gnome.desktop.interface cursor-size 96

The 96 at the end of this command appears to be the largest size that works when I test this on Ubuntu 20.04. You can also set this value to 24, 32, 48 and 64.

Now I'd also like to address your question asking if you might have a better experience using a different desktop environment:

TL;DR: Other desktop environments can offer more fine-tuned customization at the expense of a uncomplicated and consistent UI experience.

The GNOME3 desktop used in the "vanilla" flavor of Ubuntu has advantages and disadvantages for sight-impaired users.

For its benefits, GNOME tends to have wider margins around text. This makes things easier to read. GNOME devs have also put a lot of effort into making the desktop a consistent UI experience. That consistent experience means that nearly all of the time, you can expect to find UI elements in a consistent location without having to scan the whole screen.

For its disadvantages, in comparison to other Ubuntu flavors, GNOME generally has fewer customization options for the end user using the built in "System Settings" menus. This means that it might be more difficult to fine tune the UI to exact specifications.

Just using one example of this, in GNOME3, there is an ON/OFF option in Universal Access for "Large Text" which makes all of the text a little bit bigger. This is the extent of the user options to adjust the appearance of text using the built in System Settings. That's not to say that this is the end of the road with GNOME; there's additional software like "Gnome Tweaks" that add additional functionality to system settings, and there are often ways to further customize the desktop with terminal commands or by creating configuration files.

By comparison, In Kubuntu (KDE), the default "System Settings" model under Appearance > Fonts allows you to set the font and its size, style, and weight-- and you can set these separately for each of the following UI elements: General, Fixed Width, Small, Toolbar, Menu, Window Title. You can also customize the font DPI and you can also fine-tune font anti-aliasing or turn it off completely.

As for the cursor settings in KDE you can actually choose a cursor theme. Cursor themes can support multiple sizes. The two built in cursor themes (white w/ black outline and black w/ white outline), they only support sizes of 24, 36, 48, as well as "resolution dependent". The cursor is not as big as the size 96 in GNOME, but there are currently 959 cursor themes in the KDE store. It would be fairly tedious to go through them, but if you want to find the perfect cursor, you've sure got a lot of options.

In my experience, the most accessible workstation is the one tailored to the abilities of the specific individual user.

What I would definitely recommend is that you "TRY" out some of the different flavors of Ubuntu before you commit to anything. You can boot from USB installation media for any of Ubuntu's flavors and use the "Try Ubuntu" option and it will not affect your existing installation. This will let you experiment with different applications and see what kind of features they have that you could benefit from.

Here's a link to the official "flavors" of Ubuntu:

https://ubuntu.com/download/flavours