not knowing what engine or ignition you have its a bit hard to give a good answer.

But by the assumption that your using a coil still you need to find an electronic coil ignition tachometer which can be found in many different stores i.e auto meter

In regards to size its best to remove your old one and measure the back mounting and also the front face and thus go store hunting online to find something that meets your requirements there are so many different designs to choose from.

I personally changed mine to the GX tacho as it registers to 8000RPM and has redline limits in the tacho it looks alot nicer :)