Are you putting in a mirrored recessed cabinet over each sink? You might try cutting out brown paper in some of the standard sizes and taping them on the wall. Try finding the style of cabinet you like first on-line to see what sizes are available. It also depends on how far apart the two sinks will be (there is some optimum distance between two sinks) and how wide they are.

5" height for drawer openings. 1.5" framework in between. Height of door openings will be deterined by how tall your bases are (typically 34.5) and the height of your toekick which is anywhere fro 3 to 4.5". Some vintage looks have no toekick at all.

My pulls vary on the uppers, from a 3" bunny head to a 5" bar pull. Some just have fat knobs. I personally don't like the look (unless it's a really modern kitchen) of long bar pulls on the upper cabinets. It's not like you need to grab several inches to open the door. My cabinets are shaker (modified style but close) and go up to the ceiling. The pulls I have are bulky but not super long (which would look awful with the other pulls in my kitchen).

You can buy a mirrored medicine cabinet that has integrated LED lighting, which can also offer a nightlight with dinner function. So, you will not only have lighting from your main bathroom, but an additional light source. As for size, you can also get customized sizing where cabinets can be ganged together. Two 12 inch cabinets ganged together or two 15" ganged together would work where you can add lighting as an option and have interchangeable left or right open doors. It gives you alot of storage over your vanity without looking too wide. And if you want to spend the extra cost, you can have them recessed mounted as well. This will make them look flush with wall and less bulky over your smaller vanity. I hope this helps.