So, I've been running nothing but theater of the mind with 5e lately, and just writing down some thoughts...
When you’re playing in theater of the mind mode, without a grid, and you’re using abstract distances—near, far, close, it might seem like you will eventually run into measurement problems or rule disputes because ranged weapon distances are exact, for example. Or if you’re playing with grids and minis, you may find yourself wondering how to resolve coverage issues when a spell area is spherical. Using your trusty AOE template you lay down the orange acrylic circle to show who’s in range, and that’s when the rogue says, “I use my reaction to squeeze into the far right corner of my grid square, just out of the arcing range of the fireball. It’s a little warm, singed a few tassels, but I’m good.”
Maybe treat the accuracy of the text that defines spell rules, capacity, and damage the same way you would treat the accuracy in a text from a world without standardized forms of measurement, or accurate language translation, a world without an established scientific method or commonly used experimental practices or processes.
The same goes for monsters, but with the additional complexity of organic growth, the varying effects of environmental and nutritional differences on development, and the fact that most creatures we’re going to meet will not be clones, perfect copies of the one monster whose magical abilities, combat skills, tastes, and responses were recorded for the stat block in the Monster Manual. A beholder’s central eye creates a 150-foot cone of antimagic, so it can be assumed that a long time ago, someone very bold got out a measuring tape and rolled it out until the magic started functioning. But the 150 feet that daring individual wrote down only applied to that one beholder. Why would it be assumed that every beholder has identical powers and identical range? This is the natural world, exceptions to every rule are common. --This is what's cool about the random beholder tables in Volo's!
This kind of thinking and testing the boundaries of the rules always makes me think of D&D being “infinitely flexible”: PHB page 5: “The DM determines the results of the adventurers' actions and narrates what they experience. Because the DM can improvise to react to anything the players attempt, D&D is infinitely flexible, and each adventure can be exciting and unexpected.”
And of course, that evoker or displacer beast isn't going to let you mark attack ranges in chalk on the dungeon floor. Instead, the first player to whip out a ruler will probably lose that arm up to the elbow. That’ll keep the adventure “exciting and unexpected.”