Creating a VK application. How to properly size a Canvas?

Question

HTML

The task is to write the client part of the game for the social. VKontakte networks. The whole thing should be in HTML5 + JS. I started to understand the examples and I was confused by the following: the dimensions of the Canvas are set there by two constants, but then what about different screens? How to correctly set the size of the canvas?

Answer 1

Specify dimensions in relative units, not:? Why constants?

Answer 2

In VK, in the settings, the dimensions of the application are set.
The maximum can be somehow perverted with the height, the width cannot be changed from the application.