When you use VoiceOver for example, an easy test is dragging your finger down the middle of the screen, you should be able to access the majority of the content - when the menu is tucked too far up in the corner, or too small, it gets obscured by the logo, CTA or anything else that's in the area and gets bypassed by VoiceOver.

Making it large enough to find by simply dragging your finger down the middle of the screen is a good enough test to determine if its big enough, too small or located somewhere it would be skipped by those screen reader tools.

Hope that helps.