Start with the distance from your couch to the wall. As a rough working rule for 4K sets, take that distance in centimetres and divide by four to get a comfortable screen size in inches. A 2.4 metre viewing distance suits a 58 inch panel well. Sit three metres back and 65 to 75 inches starts to make sense.
HD sets are a different story. On a 32 inch HD screen you want to sit closer, around 1.5 to 2 metres, otherwise you lose detail. That is why the 32 inch works so well in a bedroom and disappoints in a big open plan lounge.
The second thing to check is the wall itself. Face brick and concrete take a bracket without any drama. Drywall needs the bracket anchored into the studs, and if there are no studs where you want the TV, the installer must add a backing board. Tell us the wall type when you book and we will bring the right fixings.
Last point: leave 10 to 15 centimetres of clearance behind the screen for the cables and the plug. A flush mount looks beautiful until you try to plug in a soundbar.