Your requirements don't appear to be very specific:
- What OS (Windows, OS X, Linux, or other);
- Which platform (PC, Pi, Mobile device, mini PC, MCU);
- Which processor (Intel, ARM, Atmel);
- Which size (0.94" up to 14");
- What resolution;
- Which input connectors (VGA, HDMI, DVI, USB)
- Most importantly - cost
So, I will assume that you are fishing for possible solutions.
There seem to be three simple options, depending on what, and how, you wish to want to achieve your objective:
Use a mobile device
As you say
I thought to create an app
assuming that you mean a mobile device (iOS or Andriod) app, then, depending on the actual size of display that you require, you could use a second hand, iPad, which are cheap and readily available on eBay. If an iPad is too big then an old version of an iPod (an iPhone would be over kill, unless you need a 3G connection to your server, which I doubt) would suffice. If the app is for Android, then a facsimile of an iPod/iPad would satisfy your needs, a second hand Galaxy Note or Tab, for example.
Use a small monitor
If you mean an application, rather than an app, for a PC based OS (OS X, Windows, or Linux) then taking your question at face value, the best (i.e., cheapest, smallest monitor), would be one of the monitors supplied by Lilliput, such as the 10" 859GL-80NP/C.
For cheapness, you could use an old, vintage, CRT based Apple Color 12" Monitor, which has a resolution of 512 x 384. You would require an adapter, from Apple's monitor connector to VGA, but these are easy enough to find. You may, nowadays, have trouble finding a video card that supports it though.
Use a Lapdock
A Motorola LapDock 100 has a small screen, and accepts HDMI, which is fine so long as your PC has a DVI or HDMI output. Granted it also comes with a keyboard, but it would be useful for testing purposes, and could be used in conjunction with a Raspberry Pi - known as a PiDock. A prefect testing ground for your project, assuming that you are using a Pi. If you are indeed planning on using a Pi, then I would suggest the Motorola Atrix LapDock is a better solution than the Lapdock 100 as it has more and better features. Avoid the Lapdock 500, even though it appears on paper to be more fully featured, as it has some odd quirks that make it difficult to use.
To my mind, this is your best option to prove your project, as it would only require a simple set of Python scripts to get it up and running, and should you, thereafter, want to cancel/scrap the project, you can use the Raspberry Pi and Lapdock for some other purpose, and hence save money.
Build a custom solution
If you only need to display five numeric characters then a monitor seems to be overkill, for both a cost and energy efficiency aspect. Much less power hungry, and arguably more of an interesting project would be to create a custom solution, using an Arduino and a Wireless or Ethernet shield (with the appropriate sketch to query the server) and an LED display, be that a 0.94" OLED, or a set of five 6.5" seven segment displays, or anything in between (again depending upon your size requirements).
If you can hammer down your requirements to be more specific, I can edit my answer accordingly.