Make the default image larger

[sindresorhus]

A lot of people have high resolution (retina) screens and default size becomes very small when use make it use half the width (to get double the resolution).

Like here: beatfreaker/text-meme-cli#1 (comment)

[beatfreaker]

@sindresorhus so the default size should be 160 right?

[sindresorhus]

No, the double of the size you want to use as it needs to be shrunk by the img tag to half to the size to be retina. So if you want it to be 160px it needs to be outputted in 320px. I think 300px would be a good size, so it needs to be outputted with 600px.

[beatfreaker]

changes default image size from 320 to 600 check here
@sindresorhus can you please review it once?

[sindresorhus]

Looks good, but you should also document the default size in this readme and in the CLI readme.