Hi Natalia,
In free theme, the logo image size is restricted to 100×50 pixel.
In PRO theme you can use any size of logo image and you can also change the image height and width to suitable to your website header section.
Please check the documentation for using the logo image in MilestoneZ PRO theme
https://www.gracethemes.com/documentation/milestone-doc/#site_name
Regards
Grace Themes