Well, each person has their own reading strategies and whatnot. Some people read lightning fast and retain all of the information. Some people read really slow and don't remember a thing! So it is hard to really give "good advice" on how to improve RC. I think the only advice I can give on RC that is 100% proven to be helpful is to just keep reading scientific articles and keep taking practice tests!
I employed the kaplan "road map" strategy, and it really helped for me. I am not a great RC'er, so I am extremely happy with my 22. For each passage, I will try to give 12 minutes for reading/road mapping, and 8 minutes for answering questions. Writing lightning fast like someone is behind you with a gun REALLY helps. As long as you can read your writing, that is all that counts, so make up your own abbreviations for long words that come up often on the science passages.
And to answer your question on the length of the passages, I think each were around 15 paragraphs. Nothing short, but nothing ridiculous.
