Are the shelf support brackets used on RTA furniture all the same size?

No, the most common size is 5mm, but there are many variations. If you can't find the right size, it may be easier to just drill the holes out a little larger to a size bracket that you can find.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.