I have a Mac Mini, the previous generation, and I'm running dual screen on it however, due to circumstances, I am using it with a 17" and a 19" screen which is not ideal. Does anyone know of anyway I can get the Mac to recognise that they are different sizes and therefore adjust the resolution accordingly as having two different physical sized screens with the same resolution is very annoying... Thanks in advance =)
A 64" HDTV has the same number of pixels as the current 21.5" iMac: 1920x1080. It's just that the pixels on the HDTV are about three times "bigger". (Not necessarily three times bigger individually, but three times farther apart, center to center.)
If you used that HDTV as a second monitor for the iMac, when you drag a window from the iMac to the HDTV, it would get three times bigger visually. If you had a window straddling the two, the half on the HDTV would be three times bigger. That is certainly disconcerting.
You get the same situation with 19" and 17" monitors that also have the same number of pixels, which the Mac is correctly detecting. (That number, probably 1280x1024, is the "Resolution" listed in System Preferences, which is independent of the physical size of the monitor.) Instead of 300%, it's only about 112%, but still annoying.
Unfortunately the cure is worse than the disease. In theory it might be possible to overdrive the 19" monitor at 112% (i.e. push more pixels than it can actually display). Let's see, that's 1434x1147. If you could do that, then a window (or anything) would be the same visual size on both monitors. So you've "solved" that problem. But now you have three new problems.
As you might surmise, 1434x1147 is a totally oddball resolution. You'd need custom driver software to change the timing of the video hardware. Maybe you couldn't get exactly that, but something close. It would have to preserve the 5:4 ratio -- and as an aside, who's the genius that decided to have a single "standard" at that ratio -- otherwise everything would be stretched/squished. And if it's not that resolution exactly, instead of being 12% off, you might be only 5%, but is that really enough? Where do you draw the line?
Second, monitors tend to dislike non-standard resolutions, especially ones that exceed their native resolution. There's a good chance the display will be garbled, or you might see nothing.
Finally, running an LCD at anything other than its native resolution causes the pixels to be fuzzy. (At a smaller integer subdivision, like a 1600x1200 monitor at 800x600, it's blocky instead. At a smaller non-integer subdivision, like at 1024x768, it's fuzzy and blocky.) Overdriving the monitor might even cause individual pixels to disappear. In short, the picture looks worse. This might be "acceptable" if you're watching video, where you're dealing with compression artifacts and pixels constantly changing anyway, but for everything else, especially text, it mostly causes eyestrain.
So to summarize: it would hard to get it to work; it is unlikely to work; and if it did work, you very well might not like it.
I presume you have a Mac Mini with both a Mini-DVI and a Display Port. You can use the displays panel in System Preferences to set the resolution of each monitor independently, a separate panel will open on each screen when you click on the Displays icon in System Preferences.
Ensure "Mirror Displays" is NOT checked under the "Arrangement" tab of the primary monitor. If you are still unable to set the resolutions of the displays independently, then more information is necessary. What adaptors are you using to connect the monitors to the computer? What specific models are you using?
Also, @Daniel Beck may be correct that the native resolution of both displays are the same. Many 19" monitors have the same resolution as 17" monitors but with bigger pixels.
EDIT: The Native Resolution Wikipedia Article has a pretty succinct explanation. If I understand your question correctly, you're looking for different resolutions for purely cosmetic reasons. You shouldn't worry about whether the size of the screen "looks the same" in System Preferences, or whether the size of the pixels or screen elements "matches" or not. That's never going to work. What matters is the optimizing the quality of the image displayed, and that only happens with LCD monitors when they're set to native resolution.
If both screens (I assume LCDs) have the same e.g. 1280x1024 resolution, and the 19's pixels are bigger than the 17's, that's the way it is. Any alternative would make you worse off, since the quality of the image is severely reduced by non-native LCD resolutions. Don't buy 19" displays in the first place.