Most of us set up our pages with a 12 point font, probably Times New Roman. If you find that this is hard to read, you can easily enlarge it. No, don't go for 14 or 16 points! Instead, "zoom" the screen until it is a comfortable size. Then you won't need to remember to change the font size back to 12 point before you ship off the manuscript. The change won't affect printing, either.
To zoom, go to View. This is somewhere in your top toolbar. In more recent versions, a slider bar can also be found in the lower right corner. See what Zoom is set to and make it bigger. Inspect the results and adjust as needed.
And, by the way, no where is it written that you must draft in double line spacing. Instead of 2, try setting line spacing to 1.5 or even 1.15. I like this because I can see more text on the screen. This change is one you do need to remember to fix before you submit!
This tree kangaroo wants to read over your shoulder.