The font size of the search popup is dependent on the size of Environment font (under Tools > Options > Environment > Fonts and Colors), but as well considers the current zoom of the editor window.
If you don't want to increase the Environment font size (because it affects as well the menu size, etc.), you can change (decrease) the size of the Text Editor font and then change the zoom of the editor window (using Ctrl+wheel). This will have the side-effect of increasing the font size in the popups.