From top to bottom: normal GUI scale, small HUD; normal GUI scale, proper HUD; large GUI scale, small HUD; large GUI scale, proper HUD; auto GUI scale, small HUD, auto GUI scale, proper HUD. None of them are quite right. My monitor resolution is 1920x1080, standard 16:9, if that makes any difference.

This is exactly as designed. The new GUI uses the same scale as the MC GUI. If you've got your GUI settings set to the tiny Normal scale instead of the Auto settings (which will set you to Large scale due to your monitor's resolution), then for sure it'll be too small, as it's the same size as the inventory GUI. I've seen the sizes y'all have for that GUI, and if you want a small inventory screen, you get a small HUD too.

As to width, it will never stretch to the edges. Doing that would cause texture stretching and would mess up the aspect ratio of the GUI. The whole point of adding the ability for custom GUI textures is to allow for set instrument, switch and button positions in the GUI. If I make the HUD/Panel cover the whole width, that won't apply due to differing monitor resolutions.

TL:DR It's working as intended, and I ain't changing it.

I just thought it was weird since the old one didn't do that. I can't play on any GUI scale other than normal because it takes up my entire screen and looks dumb, so I guess I'm JSONing my gauges to be optimally positioned for that on the transparent GUI...

Do you have a picture of your inventory size in normal and large mode for reference?

Yeah, give me a minute to launch the game. Also, as another note, the auto GUI setting is actually bigger than large in my case.

Normal first. Uses up a good amount of screen space to make things visible without making you move your mouse too far or lose awareness of what's around you. Auto second. Gigantic, takes up way too much of the screen and, due to the way I hold my mouse, actually makes me reposition my hand to move all the way across without bending my wrist too far. There's a reason normal is called that. And, as a side note, using normal GUI scale makes each item slot in the hotbar exactly 32x32, which is great for using MTS to auto-generate item textures.

Yeah, the larger GUI is what I usually use as I can't see the smaller icons on my smaller screen. Not to mention my mouse speed is rather fast, so a quick wrist flick gets me all the way across the screen. That being said, the panel has all it's buttons, switches, and items to the same scale as the GUI, so all switches will be the same height as the arrow buttons, and all instrument item icons will be the same size as the item icons in the regular GUI. So really it's just scaling it to match what you have set other places.

EDIT: For me, Auto scale in windowed mode makes items 32x32, so perhaps it's the up-scaling on the full-screen resolution that makes you use normal mode while I use Auto-Large.

Your screen's definitely bigger than mine, remember I'm on a laptop (15.6-inch and sitting desktop distance away from it). Though now that I'm so far away from it I may have to consider making the GUI bigger... though I can't stand large. I think its scale combined with my screen doesn't line up right, because it looks blurry. I have to do either normal or auto.

And about the fullscreen resolution, I have my game set to always run at native 1920x1080 even if it's windowed, so that's why. If I played in the default 640x480 then I'd have to use large, anything else would start to entirely lose clarity.

That's fair, my desktop's main is a 1440x900. It's what got me using normal instead of large, since large is huge on a lower-res screen. It just bothers me a bit that the HUD is so tiny, you can't force it to render at full size or something? I don't think anyone would be bothered by that, we'd all prefer a bigger HUD as far as I know...

My screen may be larger, but the resolution is smaller. My max resolution is 1440×900 with a 16:10 aspect ratio. It's kinda an odd setup in the world of 16:9 1080 setups we have nowadays, but it means I develop more for base systems via windowed mode and odd resolutions than supporting only 1080 systems. That's one of the reasons the HUD and Panel don't extend the full width of the screen.

Ah. Weird. That's unfortunate for people who use the normal GUI size...

Well, we shall see what happens. If enough people complain my hand will be forced.

Negative. I could force it to be larger, but then when you would change the MC GUI settings it would also change the scaling. There's not a good way to render GUIs like HUDs and Panels size-independent of the main game without fouling up the aspect ratio or scaling.