Is it possible to make the map size a combination of % and pixels.

1) When i define 25% for a map (= 320) pixels it is for most screens enough. But when a user has a big screen with 1600 pixels the map becomes too big. It would be nice if there was a possibility to set a absolute limit. In this case 300 or 320 pixels.

2) The same for the minimum.
While a percentage  is good for most circumstances sometimes a absolute minimum would be nice.

3) Is it possible to change / define your own map function.