Some GPS gauges will allow you to strech them ???
To find out if you can just move your mouse slowy along the border of the gauge and look for a set of bars to appear on each side of the border then just click and drag the gauge to increase its size. This will work if you had to click on a icon on the panel to open the gps gauge with. If the gauge is a part of a panel the only way you can change its size is to edit the panel that it is in.

To change the sizes of "fixed" gauges (as opposed to popup window gauges, as garymbuska has suggested) you will need to edit the aircraft\panel\panel.cfg file.

To me the easiest way is just get (free) CfgEdit http://www.cfgedit.com/

A harder way is to edit panel.cfg with notepad.  Here's a typical gauge entry:

gauge88=mooney_bravo!Clock,  4,240,110,54

The gauge number isn't that important.  Just number them consecutively and make sure that 2 of them don't have the same number.

The first 2 numbers will tell you where the upper left corner of the gauge wil go when superimposed on the panel's background graphic.  The second 2 numbers tell you the size of the gauge (horizintal, vertical).  If you don't want to go the CfgEdit route you can mess around with these by trial and error and see what happens.

If you're experimenting with notepad you could double the size of this gauge, REMark out the old gauge with a couple of backslashes (just in case) and change the gauges configuration to something like this:

//gauge88=mooney_bravo!Clock,  4,240,110,54 <- REM out existing gauge
gauge88=mooney_bravo!Clock,  4,240,220,108 <- Double gauge size