If you don't know, the kindle lets you set larger font sizes through the menu options. They're not massive, but a lot bigger than default. Some kindle models also have text to speech to help with the menus and read text out loud (although I don't personally think it's great.)

Seeing a low vision doctor may help her get the tools to be able to read more easily. There are lots of things they can recommend based on her condition, such as glasses, full page or stand magnifiers, contrast enhancing tools, or electronic magnification systems (like CCTVs.) This [article]
(https://nfb.org/Images/nfb/Publications/vod/vod216/vodspr0705.htm) is aimed at seniors with diabetes and vision loss, but contains a lot of useful information for any senior with vision loss.

An ipad might be a better way to read, as she will have access to more font options, brightness and color controls, as well as specialized apps (like voice dream reader) designed for large print and audio use.

There's more ways to acces reading with audiobooks than ever. If she has (or can get) a smartphone or tablet, mainstream apps like Audible are great, but there's also several free or cheap services designed for people who can't read regular print due to a disability, like https://www.bookshare.org/ and https://nlsbard.loc.gov/login//NLS (in the US.)

An Echo (or similar device) can also be used to access kindle books and audible books, as well as numerous other sources.

Local libraries often have selections of physical audiobooks, but if her doctor certifies that she has a disability which limits her ability to read print, she can also qualify to get them by mail order for free in the US. https://www.loc.gov/nls/ Many other countries have similar systems.

There's also [NFB newsline]
(https://nfb.org/nfbnewsline) a free service (with proof of disability.) You can call in by phone to use it, or download the app for android/ios (and several other ways.) They have hundreds of magazines and newspapers, as well as things like TV listings.

There are also state and local resources that help [seniors]
(https://nfb.org/seniors) adapt to vision loss. If she has trouble reading, she may need help learning how to carry on with other ordinary tasks in her daily life, or ongoing assistance. Your state's "aging agency" or department of rehabilitation might be the first places to look to get those services (if she isn't already.)

Some of the above assumes she is in the US, if not, please let us know where if you want more specific info.

Consider audiobooks such as those sold by Audible.com

Depending where you live their may be other sources of audiobooks.

Get her a big magnifying glass with light! They make square ones that you can rest on the page you are currently reading so it doesn't tire your hands that much (nor your eyes due to the distance staying the same) . I am sorry I can't describe it very well but basically it is like a magnifying box. If your grandma isn't particularly tech-savvy a Kindle or different e-book reader might be difficult to get used to. If it is going to be a Kindle, get one with real buttons (I think the older models have this). If you can, have her try it out before buying. Also try to figure out if she has some colors or contrasts that are easier to see and go from there. Maybe your local optometrist can help, too.

I'm guessing you're from the UK, so I'd recommend getting her in touch with the RNIB and other organizations for seniors, if she's willing to look into it.

As others have said, iPads generally are better at magnifying, but most kindles can magnify up to 2x if I remember properly.