We've preset the grid size to Roll20s grid size, so no changes need to made there in order to fit the Roll20 grid.
You only need to make sure that the file size of your exported map is below the upload threshhold of Roll20s upload limit.
First you will want to export your map as jpeg, or png, then you need to choose the level you want to export.
If your exported map is above the upload limit, you will need to enable tilinig in order to be able to upload your maps.
Don't forget to click the apply button in order to save the changes for the export
Therefore you click the tiling switch in the Export Size section of the panel and select the amount of tiles you want to split your map into.
Then, just hit the export button and save all the images