The logo you upload can be any dimension you like, but when displayed on the status page we'll limit it's height to 72px.
For most logos though, somewhere round 30-40px high looks best. (we use 32px for our own logo)
Dealing with high-resolution screens
You may find that on high-resolution screens your logo looks a little blurry. The best way to solve this is to use an SVG format, which can be scaled to any size without losing it's clarity.
However, If you don't have access to an SVG, I'd suggest doubling the height of your original image to 144px, so when it's shrunk to the 72px on the status page, it will still display clearly to those viewing on a Retina screen.
If you'd like more help, send a copy of your logo to us, and we'll be happy to help with getting the size right.