#1 Scrollbar Size:
I was trying to keep it consistent with the nomenclature used for Toolbar and Tool Window buttons. Either way, people are going to try different sizes until they find what they like.
#2 Document overview bar size options.
True, however, it does get larger along with the scroll bar for larger scroll bar sizes.
There are only two sizes though. If enough people want a wider document overview bar in combination with a narrow scroll bar, I'll consider it for a future release. In practice, the height of the document overview bar is more of a limiting factor than it's width.
#3 Show Block Matching
If you have Tools > Options... > Editing > General > Highlight Matching blocks enabled, then the block matching will show a marker for the beginning and end of a block. For an example, try putting your cursor on the open brace of a large function body.
#4 Annotation mouse-over popup.
You are absolutely right.
#5 Multiple marks
Also true.
I will look into #4 and #5.