It requires modding. There is a (hard to read) tutorial on GameBanana; I'll try to see if I can't make a 2x and/or 4x resize of the vanilla HUD.
Update: Well, I resized the crosshairs by 2x, but the rest sounds like it'd be a lot harder. Here are some resources of note: https://twhl.info/thread/view/14850?page=last https://gaming.stackexchange.com/questions/363103/how-can-i-make-the-ui-bigger-in-half-life-1