In cam_ext.cfg, there's the following section:
; enable scaling of in-game 2D overlay (HUD), makes the things like current item/weapon text, or SS2
; HUD/panels, larger at resolutions of 1280x960 or higher with pixel perfect scaling (requires "use_d3d_display")
; NOTE: Enabling scaling may actually improve performance, especially at high resolutions.
;d3d_disp_scaled_2d_overlay 16
; alternatively a fixed 2D resolution can be defined, which is aspect ratio corrected and scaled up
; to the current resolution, without enforcing pixel perfectness (ie. filtering makes it a bit less crisp)
d3d_disp_scaled_2d_overlay 640 480
The values of the two d3d_disp_scaled_2d_overlay lines may differ in your file, but try uncommenting one or the other (by removing the ; at the beginning) and experimenting with different values. "d3d_disp_scaled_2d_overlay 640 480" works well for me, it makes all the HUD text comfortably large to my eyes, but ymmv.