If a server rack accessory is listed on a server rack's product page under the Accessories tab, it will fit in the server rack.
Server racks offered by StarTech.com have an industry standard width of 19 inches, but can have varying height and depth. The mounting depth is important when you choose a shelf to be used as an accessory. Shelves that mount on all four corners of a server rack, such as 4-post server racks and cabinet server racks, will all have a maximum mounting depth. Many of these server racks and shelves are adjustable, which means that there will be a minimum and maximum mounting depth. This information can be found on individual product pages under the Technical Specifications tab.
It is important that the depth of the shelf is compatible with the server rack's minimum and maximum mounting depths so that it can be mounted. If a shelf only mounts on the front on a server rack, the mounting depth is not relevant as long as there is enough clearance behind the mounting point to accommodate the full depth of the shelf.