Select 'Create a design'.
Select 'Custom size'.
Enter 300 x 200 px and click 'Create a new design'
Then select 'uploads' from the left-hand nav and upload your .png logo file.
Once uploaded, drag and drop this onto the canvas.
Resize your logo to fit in the space by clicking your logo and dragging the corners.
Download your file by clicking 'Share' in the right-hand corner. Select .png file, making sure to select 'transparent background, then hit ' download'.