Your specific link says Inlet: 2" (which is one dimension you look for) and it includes Universal Fitting kit (50cm Flexi pipe, Reducer, Clamp and Strap). This means you probably wouldn't need to weld anything. It also says it fits your car.
General Advice
Look for inlet diameter, outlet diameter, dimensions of muffler, and position of inlet and outlet ports.
Inlet diameter is the size of the pipe nub on the muffler. The existing pipe under the car should already be expanded to fit over this nub. This expansion is usually just a hair over the required diameter size and ~5" deep. An exhaust clamp clamps around the two.
Outlet diameter only applies when the muffler is more hidden under the vehicle and a tail pipe (separate piece) is used to route the exhaust out the back. The same type of expanding and bolting together apply here. The tool to expand this is quite simply called a tail pipe expander.
Dimensions are the space available under your vehicle. You can start by measuring the existing muffler and seeing if you have free space in any direction. Leave 1" of clearance from everything around it. Mufflers get very hot.
If the muffler is not specifically designed for your car, you will need to fabricate and weld some type of hanger bracket onto the muffler. That sounds complicated, but is literally: bend 1/2" thick rod in weird way, weld it to a strong point on the muffler. Some vehicles and mufflers are designed to hold the muffler with a long band of metal clamped around the outside of the muffler. This is the much simpler method.
Always buy a replacement muffler clamp. Your kit includes one, but it should be noted. There is a very high likelihood that you will destroy any and all bolts that are attached to your exhaust when you mess with them.
If your buying a muffler and tailpipe separately, you need to buy a muffler clamp for that too. Your link is for a muffler with an integrated tail pipe, but not all mufflers are like that so sometimes you might want to buy a new tailpipe also (for looks or whatever).
You will probably also want to purchase new rubber exhaust hangers. They can break (go missing) sometimes.
