Hi Yen,
The easiest way to change the toolbox size is probably through CSS.
This page lists some of the classes that control the styling for the toolbox but you can also use the inspector in your browser to test out different CSS changes.
For the background color, you can change the blocklyMainBackground style, but note the fill attribute will only work when there's no grid (the grid works by applying a pattern fill).
Cheers,
Erik