Allow size control by line count (height) and character count (width)

[dusktreader]

Is your feature request related to a problem? Please describe.
I love with vhs that you can declaratively define what you would like to show. However, specifying the with and height of the output is non-intuitive. It's a frustrating of setting sizes, producing a gif, tweaking sizes, producing a gif, tweaking sizes...

Describe the solution you'd like
It would be much nicer to be able to define the size of the output in terms of the text output. It should be possible to set the height by the number of lines and the width by the number of characters.

Describe alternatives you've considered
I've used Width and Height to tinker with the output size, but it's not a lot of fun.

Additional context
asciinema provides size controls by "cols" and "rows": https://docs.asciinema.org/manual/player/options/#cols