I have an Asus Transformer TF101, with a 10" screen. I can type at ~65 WPM on it. That's about 10 WPM slower than on a desktop keyboard. As far as I am concerned, that is perfectly usable.
Having said that, at least one Android maker has built one with a 13" screen.
I've tried out the TF101 and the newer Prime at Best Buy, I also owned one of the earlier Asus Netbooks the EEE PC 901. I've also used a couple of iPad keyboard cases.
My hands are too wide to set all of my fingers on the home row in a natural typing position, so I was forced to look at the keyboard and to use a hybrid-touch / hunt and peck method on all of these devices. For this reason, I personally don't find them usable. I don't think my hands are freakishly large, so I think that many other people would have this same issue.
I looked around for the 13" tablet. I had never heard of it before but apparently it's a Toshiba.
Having said that, at least one Android maker has built one with a 13" screen.