According to Action Bar Icons action bar icon for the mdpi screen should be 24 x 24 px but the icons in Action Bar Icon Pack for mdpi are 32 x 32 px. The sizes do not match for other screen sizes neither. So which one is correct and what dimensions should the action bar icon have?
UPDATE: this answer is no longer valid, see the below answer for more up-to-date guidelines.
I believe they have to be 32x32dp, but the actual image itself should be 24dpx24dp centred. The Android design website has the correct guidelines.
I've submitted a bug report about this but have yet to hear anything...