Thank you for your interest in the Aspose.GIS product.
Dynamic map rendering size is possible if you can specify the cell sizes.
The size of a map in pixels and the cell size per pixel are interdependent, they can always be recounted among themselves. One of them is always required to build a map. We will consider the option to support the second scenario too. We have created a ticket GISNET-555 and will update you here as soon as additional information is available regarding this.
To successfully draw the overlap, You need to match the spatial reference, extent area, and the cell size (degrees per pixel) between google and your generated maps.
For Google maps, the spatial reference is usually a Web Mercator. Maybe you always work in a known area extent then you need to explicitly specify this extent. Our API allows you to specify an extent and a spatial reference for a map instance.
It is possible to calculate the width and height of the map if we get known the cell sizes for the map over which you are drawing. What is the provider your google map? If your use XYZ-tiles google servers, each z-level has a pre-defined cell size. Please share what maps are used and we will try to help you more.