My Dad has had issues typing on his smartphone most of the time... mostly the keys are a little small for him. So I checked into some different options and found one that I'm now going to even use myself. This is for an Android but they may have an iPhone option as well...
The keyboard is called Swiftkey. It has 5 different size options, 3 different layouts, a bunch of different themes so you can pic from different color options. You can choose to show the numbers above the keys or not and one of my favorite things are the symbols are shown above the keys so you don't have to hunt for the odd symbol you may not use often. There are also options for vibrate ( or not ) and clicking sounds ( or not ).
Here is a screenshot of it using a theme I sort of like ( so far )...
Dats cool craps dere. I'd neber do it here cuz den the ole man would intracoms me all da time and I'd habes ta run away after awhiles 
