Gauge diameter... 2.5 or 3"... something easy to read from across the patio... Turn the gauge so the desired temperature is up or down or what ever is convenient to look at , so you don't have to read the numbers.... In order to do that, don't tighten the gauge in the smoker... keep it loose so it can be turned....
Stem length... 2" is OK ..... The therm is just an indicator.... It won't read the actual temp of where the meat is.... You will have to get accustomed to that.... If the gauge reads 200 and your therm on the meat rack reads 230.... Now you know... Add ~30 degrees to the door therm for the meat rack real temp....
Example.... If you want the smoker, at the meat rack, to be 250 deg. F..... set the gauge so 220 deg. F is down.... Then from across the patio, if the arrow points down, the meat rack is 250.... it's so simple even a member of SMF can do it...