Just some random tips I wish I'd seen a couple years ago...
* It's better to do outlines too thin rather than too fat. Making lines fatter is easy as, but making them thinner if you decide they're too fat is annoying. * Use the smoothing function in SAI for taking care of wobbly outlines. * Learn to use color balance and levels in Photoshop, they really can make all the difference with colors. * Ctrl+Alt+Z undos mulitple actions in Photoshop instead of just one. * Don't stick to just hard shading or just soft shading. Whether you're drawing realistically or cartoon-y they need both. * Respect the sketch * When beginning your sketch fresh use very basic shapes and avoid detail. I find this helps bring out posture and good positioning/angles. * Don't restrain yourself to rules or routines (ie; nooo it must have outlines, nooo the hair must be this color)-- it can often make your pictures all blur into one. * Snapshots in Photoshop is another incredibly useful feature for comparing what you want. It'll saves the current image into a separate state for you to refer back to later. Awesome for comparisons or insuring yourself for changes you're not sure about. * Operate at a high resolution, around 5000x5000 at 300PPI. Stops little errors showing up like jagged edges or holes in the base colors. * My recommended pen button setup is right click / space bar * SAI is pretty good and a lot of people are going to chuck its name at you but it lacks a lot of features so don't get attached to it. * Don't use fan art for references. * You can use Photoshops "Save for Web" feature to resize the export of your image without resizing the PSD. Saves a buttload of time. * In Photoshop if you hold control and hit a thumbnail in your layers box, a selection box around the contents of that layer will be selected. Awesome for shading.
I learned to paint on a 3x5 inch wacom Graphire 4, with a work area the size of an index card; not the greatest way, but i still learned it pretty effectively. I use photoshop just as a canvas, and paint directly on it with no layers. Sounds scary, but it's actually great fun.
You can try using the pen tool in the lineart layer in Paint tool Sai, which allows you to adjust a line after you've drawn it. Also setting the stabilizer higher can make smoother lines. The higher the setting, the slower it goes.
size of the tablet depends. I felt my previous one was too small so i got a intuos 5 medium, which is about 9 by 6 inches, and feels great. If you are comfortable with the small size then stick with it.
I do it freehand. The main wonder thing of digital art is the undo button, so u can keep doing the lines until u are satisfied. Dont do the lines slowly, do it with one confident stroke. You can also utilize the... I duno what its called officially but its the smoothing thing, where the program helps u to make the lines smoother as u do it. If you have multiple lines it means you are stroke the same area multiple times.
As for additional tips: Fundamentals of drawing traditionally also apply, so learn them. Practice. Experiment with different functions of the program, or multiple programs. And most important of all, have fun