Quick recommendation: Profimail (for largest print) and Aqua mail (for light on dark) are the best of an average crop.
The longer answer:
While it’s one of the most common tasks on a smart phone, there actually aren’t a lot of useable, configurable, large print email clients around. Here is a roundup of most of the more popular options. Some had no text size options and some had options for lots of individual elements, I’ve set each to the largest I could and a dark theme where available, and listed the text size in the subject when viewed in the message list, and in the body when reading the email itself:
Built in Gmail / E-Mail apps (both are the same, at least on my phone) by Google (Free)
Colours – white only
Can zoom in to 28pt (text does not reflow).
Works with Talkback.
ProfiMail Lonely Cat Games (Free)
Colours light or dark, only works in inbox, not messages.
Can zoom in to 26 point and text does reflow.
Preview windows shows by default which takes up much of the screen. When removed, viewing an email shows the body only in half the screen and the top half is taken up by the sender’s information. Body of messages black text on white regardless of theme.
Works with Talkback, several unlabeled buttons.
Aqua Mail by Kostya Vasilyev ($4.77 or free)
K-9 Mail by K-9 Dog Walkers (Free)
Can zoom to about 80pt (text does not reflow)
Colours – black or white
Works with Talkback. Senders photo reads as unlabelled button and reads “Table, table table table” a bit when reading body of message – gets a bit tedius.
Pigeon by RBD Solutions Limited ($2.99 or free with ads)
Colours – themes in pro version
Works with Talkback
Zeus Mail by The Zeus Team ($5.45 or free)
Can pinch zoom messages to about 80pt but text does not reflow.
Colours Themes apply to inbox only (not messages)
Doesn’t work with Talkback (some headings read but inbox and email body won’t read.
Kaiten Mail by Kaiten Mail ($4.99 or free with ads)
InoMail by InoGuru ($4.99 or free with ads)
MailDroid by Flipdog Solutions LLC ($20.55 or free with ads)
Colours – black or white, messages appear in white though.
Can zoom to about 80 point and SOME text DOES reflow.
Works with Talkback – There is one unlabelled swipe point for each message (time maybe?)
Boomerang: Email App by Boomerang for Android (Baydin) (Free)
Can zoom to 24 point (text does not reflow)
Colours – Various themes – Message body still appears black text on white.
Inbox reads with Talkback though can’t open messages.
CloudMagic – Free Mail App by CloudMagic Inc Communications (Free – Has Google In App Billing code but I couldn’t find anything to ‘buy’)
Mails – Hotmail, Yahoo, GMail (Multiple Mails) by Mobile Tecno Apps Communication (Free with ads)
SolMail by Sol Studio at Daum (Free)
MySecureMail by Mobile Experts ($7.99 or free trial version)
Summary: I was sadly disappointed to find so many options and none of them with any decent text size options. I was pleased to find that a lot of the apps did work fairly well with Talkback though nearly half of them had big issues with speech. Most of them you can pinch zoom in on message contents though for the most part the text doesn’t reflow, so you end up having to scroll left / right as well as up / down. Some of the apps have colour themes, but again most of them only use these in the message list, when you open a message, the text goes back to black on white. Profimail can have largish print but it’s hard to get the message to show on more than half the screen. Aqua mail is the best high contrast option though agan not perfect as the text still isnt large print.