If you want to know how to save as png I'll be glad to tell you.
SIZE
Believe it or not, MS Paint's tools do not have a set tool size. By first making sure num lock is on (activating the numeral keypad), holding down ctrl and plus will make the brush/line larger/thicker, and holding down ctrl and minus will make it smaller.
This works with the eraser tool, the line tool, the pencil, the brush (all shapes), the spraycan, the curved line tool, and on the shape tools (but only on the options with borders).
Recolor
Also a little-known fact about MS Paint, the color replacer is located in the eraser tool. To use it, set the foreground color for the color you wish to replace (the only color you want the new color to show up on), and the background color to the color you want it to be replaced with. Then, right-click and it will only draw on the color you want it to.
To try this, draw a few black squiggles on the white drawing area first, then set the foreground (left-click) to white and the background (right-click) to red. Right-click and draw around and on the squiggles. They should have remained black, but the white behind them [wherever you drew] became red.