If you are really unsure about the size of the table I would suggest getting cardboard boxes and stacking them behind your sofa. If you have several boxes especially of different sizes you can try out different lengths to see what works well for you.

Agree with doing a mockup to test different sizes to see which works best. If you have sufficient depth behind the sofa, consider a long console rather than a table for extra storage.

It would be helpfule to see other photos of this and adjoining spaces for perspective. The boxes - or painters tape on the floor — is a good idea.

I do not think covering the seams will happen and honestly if you get a waterfall type table then the back of the sofa and the height of thetable will be the same I think, so in my head you would have to look under thetable to see the seams. So how high is the back not counting the cushions? Then if you center the 90" table where would the seams be? This is kind of a cool idea where the decor takes your eye away from the seams but it depends I think on how you want to use this console

How about 2 console tables side by side?

This is the type of table I’m considering

Are you going to place some stools/benches under it? If you aren't, I wouldn't use such a plain console.