How to calculate the effective size of a 16:9 television when viewing 4:3 material:
effsize = √((diag × 0.654)² + (diag × 0.49)²)
where diag is the diagonal size of your 16:9 television.
For example, the viewing area of a 27-inch 16:9 HDTV is about the same as that of a 22-inch 4:3 television when viewing old-school, non-widescreen material.
Reader exercise: solve for diag to figure out how to replace an existing 4:3 television with a 16:9 one that provides the same viewing area as that TV.