Should the scrollbar width be thin/sleek or thick so that user can grab using mouse and scroll? I know most of them prefer using a mouse while scrolling. In terms of usability should width be thick or sleek?
Your scrollbar should be at least as thick as the smallest browser thickness since your users spend most of their time on other websites/applications and therefor expect the scrollbar to be at least as big as in their browser.
According to this post, the smallest you should go is 15px in width. This is a good middle value between usable and aesthetic and users will feel familiar with the scrollbar.
Ideally, if you have a lot of content to scroll past and if you think scrolling using a mouse wheel is tedious, you'd want a larger scrollbar.
However, scrollbars on Mac OSX hide unless you hover near them. On Chrome and firefox - Windows, you can change the width of the scrollbar on hover. So you can have, say a 4px default scrollbar and increase the width to 8px when the user hovers over it. Personally, I think this hits the sweetspot between the 2 options.