The only good exercise to practice kanji and get the good proportions is, for me, some drill practice : use graph paper (or print an Excel sheet with a simple grid and, in each box, you draw horizontal, vertical and diagonal lines) and write the same kanji several times in 2x2 or 3x3 box (according to the size you need).
To check the strokes order/proportions, you can just type the kanji you want to write plus 書き順 (which means "writing order"). If you take the first link, you should get an animation where the kanji is written properly in a cursive fashion (I don't know if cursive is the appropriate term for kanji).
For your other questions :
- All caracters that you write should have the same size to be consistent. However, if kanji with a lot of strokes are a bit larger for clarity reasons, I don't think people will mind.
- The size of the caracter doesn't depend on its number of stroke, again for consistency reasons.
- As in every language, people tend to distort caracters and create their own writing. Japanese is no exception and reading hadwritten caracters can be a pretty ardous task (personal experience). Only when doing callygraphy does people take care of writing properly caracters. Otherwise, they tend to write as quickly as possible as we would do with English.
I hope this answered somehow your questions :D