Hello Kat, and welcome!

Whwen I first studied plant drawing (many, many years ago!) I did everything in pencil – so the medium is up to personal choice – pen/ink is another good drawing medium.
I’d say 20×15(8×6″) is a reasonable size for a small study – as long as it allows you to place the plant and still include the detail which characterises the particular plant.
Try to keep it clear, to understand structure/formation ……. a sketch (for me) is usually fairly quick and catches the essentials rather than the details -a drawing is more of a detailed study……. so this, I say was more of a drawing!:)
Aim for good contrast, keeping highlights white – and those placed next to the darkest tones will provide drama and what is now commonly called ‘pop’!!

You’re off to a very good start in all aspects!

Here’s a test for you – try using a B pencil and do a block of shading from extremely light to very dark – good practice!

One thing I’ve started doing is making my sketches and studies roughly the same size as the final piece so that I don’t have to worry about messing up my proportions or perspective when I transfer to the watercolor. As to media, it depends on what preparation I’m doing. Structure or contour is done in pencil. Value and form are done in pencil or charcoal. Charcoal, especially the vine, is wonderful for large studies. Granted, you don’t get very dark, but the ease with which the material can be applied and shaped makes up for that. In other situations, where I’m trying ideas with color fields, I’ve worked with conte.

I’d second working with tonal value if you continue with graphite. Some other things to try would be varying the pressure and the place where you hold the pencil. And the 8B & 9B pencils are so much fun to work with.