Hi Hannah,
Thanks for reaching out.
First of all, try to add a lower resolution logo image ( currently it is 1000 x 470 px)
To change logo size, try the CSS below
header .custom-logo-link img {
max-height: 70px;
}
Copy and Paste it in Appearance -> Customize -> Additonal CSS
Note: you can increase/decrease the number 70 to adjust the logo size
Let us know if you have any questions
Best Regards