You could look at the Kobo Aura H2O, as it has a slightly bigger screen than the standard-sized ereaders like the Kobo Glo or Kindle Paperwhite - 6.8" compared to 6". That might be enough to make the difference with large font sizes.
There are other ereaders with bigger screens available (like the
Pocketbook InkPad for example), check out the threads in this section of the forum for some feedback on the bigger devices, but they tend to be more expensive and the bigger screen might not really be needed. I'd steer clear of knock-off Chinese ereaders as the quality just won't be the same as you'd get from the established brands.