You can't adjust the text size itself, but...
In the upper right corner of the clipboard there's two buttons, + and - with a symbol of a magnifying glass. Those will alter the size of the entire clipboard including font size. Find the size for your liking.
With Insert and Delete buttons on your keyboard you can zoom in and out on the map. The font size will remain the same irrelevantly of the zoom level of the map so zooming in will separate the field names. That is of course with the cost of losing the big picture, but in most situations you'd be more interested in just a handful of fields at a time, i.e. your home field, your target and the route between them. Notice you can also grab the zoomed map with your left mouse button to view other locations than the one where your plane is in the center of the map.
Also notice that by right clicking the map you can select which elements are shown. If you're in a fighter or a high alt bomber you probably don't need vehicle spawn points, for example.
