I have a specialized pitch, unsure of the year, and my rear wheel was stolen.
I have the front wheel which had the marking "ETRTO 584x25c" on it
What does this mean and what sizes of wheels would fit?
"ETRTO" stands for "European Tire and Rim Technical Organization" which means that's an industry standard sizing number. You can just take your wheel to your local bike shop and get them to spec a replacement rear wheel of the same size.
If you want more detailed information, unfortunately Specialized (unlike some nicer brands) doesn't appear to have an easily accessible catalog of old products to search in. However there's a site called Bicycle Blue Book (presumably based on the Kelly Blue Book for cars) where you can search for previous year models.
You're probably not interested in the valuation, but the page for each bike includes product details, so you can make sure you're getting a comparable wheel. For example, if you have a 2017 Sport 650B, the page will tell you it has "Shimano Center Lock" hubs and "[Specialized] Stout 650b, alloy, disc-only, double-wall, 25mm inner width" rims.
Note that older years are described as "650B" while from 2018 Specialized is using the term "27.5"; according to Sheldon Brown both of these are designations for (ETRTO) 584mm tires.
To replace a wheel you need to know:
Rim width and diameter we know from your front wheel: ETRTO 584x25c which means you have a 584mm diameter rim with internal bead width of 25mm. 584mm rims are commonly known as '27.5 inch' on mountain bikes.
The rim specs match the Specialized page on the latest Pitch model.
The latest model has a 135mm width rear hub, quick release and 8 speed cassette (requiring a 10 speed or less compatible hub). That's very standard for inexpensive mountain bikes.
You could go with either 6-bolt or Shimano center-lock disc attachment. obviously you'll need a matching disc. 6-bolt discs use torx head bolts, center-lock has a lock ring that accepts a cartridge bottom bracket tool.
You probably want the same number of spokes which seems to be 32 front and rear.
In addition to a wheel and brake disc you'll presumably need tire and cassette. You can get the same model as the front, or a different brand/model. You already know the rim size, width can be read off the front tire.
Cassette selection is a little trickier if you don't know the max sprocket size was. You'll know how many rear speeds you have from the shifters (presumably 8), but the max sprocket size you can use depends on the model of rear derailleur. The current model will take a 11-34 but an older model may require a smaller cassette.