I’ve been trying to decide between 0.5mm, 0.7mm, 0.9mm and I’m still not really sure which is most effective. I’ve been researching and I see a lot of conflicting info.
From what I’ve seen, for drawing 0.5mm might be best. I was wondering if anyone had any advice. Thank you!
“Effective” is all relative. Drawing? All of them are good. When drawing, sometimes you want finer lines, other times thicker ones. One of the best is 2 mm because it’s so versatile (you can use a sharpened point, or a blunt point for a wide range of lines). When I used to draw more often, I’d have a sharpened 2 mm and a blunt 2 mm as my standard, then jump down to 0.5 mm for finer detail, sometimes even 0.3 mm.
Pencils can be had very cheaply, if you just look around a bit. The venerable Pentel P20x series (P203, P205, P207, P209) is a very inexpensive way to get a highly reliable pencil set with a nice range of lead sizes.
If you’re note taking 0.5 mm is the most common. And if you like a slightly thicker line, 0.7 mm is a great way to go (it’s also more resistant to breakage).
Btw… you said you read a lot of “conflicting info” about lead sizes. What was the alleged conflict?
“Conflict” was probably a bit too dramatic! 
Some people were saying that the thin lead (0.3mm) allow you to draw finer/more intricate details. I wasn’t really sure what would be best for my style of sketching. You’re right, I guess it all kind of depends…
For sketching, I use .5mm and .7mm. Mainly because of the line thickness and frequency of breaking the lead is slightly reduced. Anything under that becomes tricky if you tend to apply a lot of pressure during the sketching.
One factor to take into account is also the typical size of your personal writing, or the scale of the drawings/sketches you plan to create.
Take a normal page of ruled paper and/or checkered paper (the distance between the lines ought to be 10mm or 1cm; the side of the squares ought to be 5mm, or 1/2cm; I’m using metric measurements as they’re the standard here in Europe), try to copy a passage from any source you might like — but make it so it spans a few lines, write on every line, no skipping — and then inspect the result.
If on the one hand the lines are jammed together, there is a lot of crossing between ascenders and descenders, and you feel that your writing needs more space to become clear (e.g. by skipping very other line), then if I were you I thought that probably a 0.7 or even a 0.9 would be a better choice, especially for note-taking. If, on the other hand, your lines are well separated on the page, and hence the calligraphy is tiny, I would go with a 0.5, a 0.4, or even a 0.3mm, so that the thickness of the pencil lines do not interfere with the structure and form of your handwriting.
Same story with the drawings, but in that case you may need different sizes to achieve different results in the drawing, shifting from minuscule details, to wide areas just blocked out quickly.
I usually work with 0.5 pencils (also because the range of available lead hardness is much larger than what can be found for any other diameter: everything between 4B to 6H can be found, at least in principle), but since I write tiny letters, I can take notes even with a 0.2mm pencil (especially if I want that nobody understands what I’m scribbling about), without any particular fuss.
Apart from that, I back Cytherian’s advice: buy the whole Pentel P20X series if you can, add up even a nice 2mm leadholder, select a wide range of lead hardness, and have fun!