I own the S4 and have played around with the HTC ONE a bit, but admittedly know virtually nothing about the stock keyboards on both. There are many settings and even custom keyboards (such as TouchPal or Swype) that allow you to modify size and appearance. I wouldn't use the stock keyboards as a deciding factor for selecting which device you buy.
I've used both and have to say, the HTC keyboard is really good. Best on screen keyboard I've ever used. I've also tried a whole host of other keyboard such as swiftkey, swype etc. The HTC keyboard has large-ish keys for the letters and the predictions are very very good.