Allow custom map size and remember json styles

[gety9]

Currently map is created at 500x500, it's fine for mobile, but we need bigger maps for wider screens.
Possibility to set size will be helpful.

Also remember json styles from previous use so it's not needed to reenter it all the time.

[highvoltaged]

Yep. +1 for this.
I wanted to create a 1240x600 map, and in this current form it's impossible.

[RohanLocus]

+1 to above issue and text labels associated with locations are disproportionate.

Great work, this plugin is very helpful and useful.