slugolicious's answer is correct in the sense that success criterion 2.5.5, which was introduced in WCAG 2.1, defines a minimum target size of 44 by 44 pixels. However, one should really take a look at Understanding Success Criterion 2.5.5: Target Size, which explains the purpose of this success criterion (emphasis mine):
The intent of this success criteria is to ensure that target sizes are large enough for users to easily activate them, even if the user is accessing content on a small handheld touch screen device, has limited dexterity, or has trouble activating small targets for other reasons. For instance, mice and similar pointing devices can be hard to use for these users, and a larger target will help them activate the target.
Touch is particularly problematic as it is an input mechanism with coarse precision. Users lack the same level of fine control when using inputs such as a mouse or stylus. A finger is larger than a mouse pointer, and generally obstructs the user's view of the precise location on the screen that is being touched/activated. (...)
A height of 44 pixels is quite big for checkboxes and radio buttons on the Web or on the desktop, which is one of the reasons why SC 2.5.5 is at level AAA: this is a level that is not recommended for all websites:
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.
However, you question was about target sizes for checkboxes on the desktop, not the web or on mobile devices. Several operating systems and widget library developers have created guidelines for desktop interfaces, but I have not been able to find recommendations for target size in any of these guidelines. Below are a few examples.
- Apple's Human Interface Guidelines contain a page about Checkboxes which say nothing about size or height; the same applies to [Radio Buttons)(https://developer.apple.com/design/human-interface-guidelines/macos/buttons/radio-buttons).
- The Microsoft Windows Design Guidelines for Desktop Apps contain sections on Check Boxes and Radio Buttons. The sub-chapter Recommended sizing and spacing recommends a height of 17 pixels for check boxes and their labels. The text does not say whether this is the recommended exact height or a minimum height. (In addition, this is only mentioned in an image and not in text or the image's alt text. Target size isn't mentioned on the general page about Controls.)
- The Elementary OS Human Interface Guidelines contain a section about Selection Controls, which does not mention target size. However, it has a link to the article 3 Ways to Make Checkboxes, Radio Buttons Easier to Click, which recommends putting radio buttons and checkboxes in a "button zone". Essentially, this comes down to making sure that the label is also clickable (just like properly connected HTML labels) and that the label and the control have the same highlight colour on hover. (Keyboard focus is not mentioned, unfortunately.)
- The chapter on Menus and Toolbars in the Java TM Look and Feel Design Guidelines, Volume 1 doesn't mention target size either. The section on Designing for Accessibility mentions font size and recommends running usability studies that also involve low-vision users, but nothing is said about target size. Volume 2 doesn't cover this topic, either.
- Oracle's documentation on Implementing Java Swing User Interfaces don't say anything about target size either. (Comments about size refer to larger components.)
