The active display area does not reach the edges of the monitor. I measure it to be about 8.75 inches by 5.5 inches. This gives an aspect ratio of 1.59 : 1. The graphics mode loses two lines and thus the aspect ratio is a sliver wider. Coincidentally, this is the square pixel ratio of CGA 320x200. There are no horizontal or vertical size or hold adjustment knobs on the monitor, only brightness and contrast. Here is a screenshot showing, somewhat faintly, the visible screen area on a 5151 :
DOSBox can take screenshots of the Hercules Graphics mode. Here is one such screenshot, untouched :
While it is a bit squashed, doubling the vertical resolution (720x696), as some Mobygames screenshots do, is just not appropriate. If you do that, you will have an aspect ratio of 1.03 :1. As you can see from the image of the real monitor, the visible screen area is not a nearly-perfect square or anything near it.
While it is not perfect, the best compromise is to increase the vertical size of a screenshot by 25% using nearest neighbor interpolation. If you resize the screenshot to 720x435, you get this result :
This gives a 1.66 : 1 aspect ratio, which is far, far closer to the ideal ratio while keeping upscaling artifacts to a minimum.
Starflight's effective resolution is 160x200, which is fairly common with older games. If you use a game that has a native 320x200 resolution, the artifacts can be a little more visible :
|Maniac Mansion Enhanced Version - Original|
|Maniac Mansion Enhanced Version - Resized|
|Leisure Suit Larry 3 - Original|
|Leisure Suit Larry 3 - Resized|
One issue to note is that most PC games use a native 320x200 resolution, and their Hercules drivers often leave noticeably large borders. Many, if not most games use double the horizontal resolution to 640 pixels, using an algorithm similar to what they would use for 640x200 CGA graphics. Starflight's effective resolution with Hercules is 640x200. The regular 16-color resolution of Maniac Mansion's graphics window is 320x128, so with Hercules they simply doubled it to 640x256. The text size remains 8x8, so there is an unused section at the bottom of the frame. Sierra did something similar with their AGI games. Their graphics window is 320x168, so with Hercules they doubled that to 640x336 and used the extra 12 lines for the status bar. Instead of keeping the 8x8 font, they used an 8x12 font unique to the Hercules driver. Typing pauses the action like SCI games, whereas it does not in all other graphics modes.
For Leisure Suit Larry 3 and other SCI0 games, the native graphics area is 320x190 in 16-color modes with the status bar taking up the remaining 10 lines. They could not double up on the graphics because they only have 348 lines to use. What Sierra did here is to increase the horizontal resolution of the whole screen to 640 and the vertical resolution to 300, effecting a 150% increase in the vertical resolution (including the status bar). Every other line is repeated. This may not give the nicest look, this was probably the simplest approach to take, which was important when dealing with taxed 8086 and 286 machines. Also, there is is no way to decrease the brightness of each individual lit pixel, so traditional filtering techniques will not work.
Here are a few more screenshots showing how non-native Hercules games fit within the screen :