I'm an engineer, therefore my knowledge of graphic design are very, very, very poor... so please bear with me.
I want to know how much space I should leave between two paragraphs for my thesis. I've heard from some friends of mine (who are designers) that you shouldn't if you are using indentation (that seems to be length=1em, according to Tschichold's Rules).
However, is it recommendable to use both indentation and paragraph space? (we call it
\parskip in LaTeX). If not, which one should have preference? What should be the length?
A DA01 posts, there is no "standard". It really comes down to what visually separates the paragraphs enough to allow easy flow without creating a "hiccup" in the motion of reading. You do not want the reader to have to stop, find the next paragraph, and continue reading. Even if that takes a millisecond, it can get distracting and unconsciously make the reading more difficult. In addition, leading and age of the reader play huge roles. Often leading is far more important than any space after a paragraph.
When I design a text-heavy piece I use a minimum of 6 points between paragraphs, usually set with a space after rule. However, as text size increases, that space after should increase (as should leading).
is it recommendable to use both indentation and paragraph space
It's redundant. So most graphic designers would say it's not recommended--pretty much because there's no need.
As for how much space to put between the paragraphs, there is no right answer to this other than put enough space but not too much.