Hopefully the client is asking for AA conformance and not AAA conformance. The W3C says that full AAA conformance is not possible.
It is not recommended that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA Success Criteria for some content.
There are some great AAA guidelines, such as 2.4.9 ink Purpose (Link Only) and 1.4.6 Contrast (Enhanced), but your client should be focusing on AA conformance which is the standard required by most country laws.
That being said, the only guideline that talks about scrollbars is 1.4.10 Reflow. It mainly says the user should not be forced to scroll in two directions (ie, both a vertical and a horizontal scrollbar) "Except for parts of the content which require two-dimensional layout for usage or meaning".
That last phrase is a bit subjective. The guideline gives a few examples of content that might require scrolling in two directions, such as maps and tables, but it's sort of up to you to decide if you need scrollbars in both directions.
Regarding target size, there is a AAA guideline that says 44x44 is the minimum size, 2.5.5 Target Size, but it also has a list of exceptions so you don't necessarily need that size of checkbox if you can fit into one of the exceptions.
When WCAG 2.2 comes out, there is a new AA guideline that says 24x24 is the minimum size. As mentioned, most companies shoot for AA conformance. Currently, WCAG 2.1 AA does not require a minimum target size so companies often don't focus on that. That will change when WCAG 2.2 comes out. However, if a company chose to follow the WCAG 2.1 AAA guideline of a 44x44 target size, then they'll automatically conform to the new 24x24 requirement.