Some Dillema

1. Look at valve maps or decompile some maps from here (for educational purpose only ).

2. Of course you can plan it in MS Paint (if you really want to use that piece of ...).
How can your drawing skills be to bad, to create some straight lines? :O

If you're having trouble with scaling, really the only thing that will help you with that is mapping a lot. You may make a ton of shitmaps, but after some time you'll start to get the hang of scale, distances, spawntimes, and other things that make a map good.

MSPaint is fine for planning, but don't think that it's absolutely necessary to plan your map before blocking it out. Some people use paper and pencil, some people use the computer, and some people just wing it and make up the map as they go along. There is no wrong way to do it.

I've done 2 maps for TF2 and thing that bothers me most are the scale of the maps. I'm still having trouble with it. Having a CS Source & L4D2 mapping background, TF2 is still the hardest to get into the "scale of things". All i can do is look the map up a lot in-game, it helps quite a bit. I suggest you do the same. Helps you keep the track of what you do without doing so much work without much of a benefit.

Loading a Sentry or class model (I usually did the Heavy) into the map temporarily while you work on it can be useful for judging scale. I used to do this a lot when I had trouble with it.

open a map of the same gametype you're making, make a block representing each area, put it in a new file, use them to dictate the scale of the area, boom.