The Company logo needs to be a horizontal format, probably about 3 or 4 to 1, e.g. 400x100, 800x200 etc
If you have a image with this ratio then it will re-sample correctly and display large

The template logo size depends on the template a client is using. The default template is scaling logo to 64px in height so that would be ideal size
If they want different size then they have to modify the following code in the template:

<img src="data:image/png;base64,${company.logo}"
alt="CompanyImage"
height="64"/>