25" 4:3 is 15" tall, which is the dimension you care about for the picture size. Minimum round number in 16:9 puts you at 31" (15.2" tall), or more practically most monitors/TVs are 32" (15.7" tall). That will at least give you the same size picture when centred.
With that said, you're now jumping from 20" wide to nearly 28" wide (plus whatever bezel/frame the panel has), so make sure that can physically fit inside your cabinet. You can use MAME artwork features to put up dynamic marquee artwork to match the game you're playing to fill in the extra display area you're not using, which can look quite nice if done correctly.
But, I'll also second the above. A CRT would look a hell of a lot nicer. At least try and investigate the option of getting that working.