To check parts and tools you will probably need some kind of height gauge for dimensions, a surface gage with indicator to compare parts with each other or with gage blocks, and a cylinder gage or granite square for checking squareness. For layout you will need the surface gage with a scriber and the height gage and some spring calipers. Not all of that stuff needs to be on the plate at once (or even ordered right away), but there needs to be room for whatever part you want to work on as well as the tools for layout or testing. You will run out of room pretty quickly on a 9x12" plate unless you only work on really small stuff. I have a 18x24", it has comfortably handled everything I needed to do so far. The biggest thing so far has been spotting my 8x16" cast iron surface plate for scraping it in. I have also calibrated straight edges up to 24". I assume you are looking at Chinese import surface plates. They can be really good, but not always due to lack of quality assurance, and there is no way to know, and no reasonable way to test it yourself. The small plate is fine for starting out, but at some point down the road you may want a bigger plate to handle bigger parts, and one of known and tested accuracy so you can calibrate your tools properly and with confidence.
Edit: The smaller Chinese plates make good lapping plates for general shop work.