A good, clear, well sized text is essential to a field worker, or someone that uses a mobile device on a regular basis. So you need to set this up to be a useful size, and also ensure the screen does not lock too soon ...

Font Size

Let's look at Font Size - when trying to read a tablet screen the font-size will impede or aid the reading progress on the screen.

To set the Font-size on Android:
Settings > Display > Font size

I configure Android devices to use 'Large' on 7inch tablets and 'Huge' on 10inch. Though of course this does depend on the user to some extent.

To set the Font-size on  iOS:
Settings > Display and Brightness > Text Size

The adjustment on iOS is via a slider, so you need to choose an applicable size. There are 7 points you can set the Font Size to on the slider. A setting of 5 or 6 (with 1 being smallest) suits most field use for the iPad.

A major irritation that you need to sort out for your staff is the screen timeout. This is a balance between saving battery life and being an impediment to getting work done.

Screen Timeout

A timeout of 30 seconds, when this is the period of time between measurements or data entry is clearly idiotic. Whereas too long a timeout and you end up wasting battery power.

For staff doing an asset inspection, where there may be about 20 key points to assess over a half hour period I set the timeout to be 5 minutes. This means there may be one timeout, possibly two during the session when the staff are involved in the inspection rather than entering data - which they judge to be reasonable.