How can I increase text size in single-user mode?

This is going to be your experience with Single User mode until Apple rewrites the underlying code to allow it to understand what retina is.

Photo courtesy of Bev Sykes

I know this is really old but I thought I might share the answer with y'all. The way Apple implements characters in the boot video mode is in pixels. There is an array of 0s and 1s that correspond to white and black respectively. Now since the Retina display has such high resolution, the density of those pixels will have a direct coincide with the size of the characters. For example, this is the implementation of capital A:

1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 0 0 0 0 0 1 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
0 0 0 0 0 0 0 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
0 0 1 1 1 0 0 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1

I hope this answers your question, and as joshfindit said, Apple would need to redesign the boot video mode if they were to fix this.