If my take is correct you want to cover that 20 inches plus a bit of the window and have them look full. If that's the case then the width depends on the type of curtain you buy and the fabric. A sheer fabric will require more width for a full look, than a heavier fabric.
It would certainly be enough if they were pinch pleated --maybe even too wide. If they were tab tops or gromet tops they may or may not be enough.

My suggestion is to go with the 54" and try them out, to see if you get the look you want. You can always take them back if they don't suit.

oilpainter, the 20" includes a bit of the window trim already. I dont want to cover any of the glass. The fabric I would like is definately a heavier fabric, like silk maybe , not a sheer type of fabric. Which type of top would look more elegant?

In my opinion you should cover all the window trim and just have the glass showing. You are trying to create an illusion that the windows go all the way to the corner and showing the window trim destroys that illusion.

Definitely a pinch pleat is the most elegant , and the closer together the pleats the better. But then it depends on the rod you have or are thinking of getting. Whether or not you have a valance box and if you have an existing sheer already.

I think you have to go looking or consult someone who makes custom drapes. Without actually seeing things or knowing the look you are after, I can only give you general advice.

Thank you oilpainter for your advice, it was great help.

Why don't you post in the home decorating forum and add a photo. I'm sure there is someone there who has done just what you are trying to do. If you don't know how to add a photo let me know and I'll tell you how

Search the web for "no-sew drapes" and see what you find.

There is a whole blog about it, but I forgot the URL.