Here is how to convert Mega Man: The Power Battle and Mega Man 2: The Power Fighters sprites to Mega Man 7 style!
Here is the comparison between arcade and MM7 Mega Man. The one on the right is wider and has choppy outlines.
Step one is to use a calculator. Select in your image editor's tool each sprite you must resize, then check the width (such as by looking at the bottom of the screen in Paint.NET once you get the select around a sprite) and multiply it by 0.75. For this standing sprite, it is 45*0.75=34. Then resize the sprite such as by crop to selection for just that sprite, then use Paint.NET's resize image function to change only the width.
The arcade sprite now looks much closer to MM7 style, but the outlines need filled in a little.
It's not perfect, but now it fits the Mega Man 7 size and style a lot better!