Question about creating wireframes and mockups in the real world of UI/UX Design

For portfolios out there I see wireframes and mockups being presented in different sizes. I was wondering if in the real world when designing a website for either desktop,tablet and mobile if you need to design to the exact same size as those screens? I know screens vary in sizes for each of the three devices. Do they just choose what the standard and majority of users use? Or does it not matter as long as the design deliverable is done and developers handle that?

Hi Ralph,
The wireframe size doesn’t matter provided the aspect ratio is correct.