We usually draw a line in the sand for what’s supported in terms of device age and design with the most common size in mind and keep in mind how it will look on the smallest supported device. Getting your hands on usage stats for things like the business web page can help make these decisions. That being said you also need to consider font scaling as well. WACG Resize Text (Level AA) dictates 200% as the scale at which you should be able to increase text without a loss of functionality. More on reddit.com

Depends what part of the design process I’m at. If I’m developing an identity, I’ll work the initial look and feel of the app alongside all the other collateral. If the identity has already been developed, then it’s a question of how well does it inform interactive elements. If I’m starting on just the “UX” of an app, I’ll be working on a whiteboard or large sketchpad to map out user journeys and flows. That’s assuming I’ve already done plenty of user research, or someone else has. At the very begging of a design, I stay away from plugins or templates because I want to feel free with as wide a solution space as possible. More on reddit.com