Can’t make my header bigger

My site hasn’t technically been launched yet, otherwise I’d link you there. But I’m using the Refined theme from Restored316 through the StudioPress site. I’ve created the header I want to use for my blog on Canva, with the dimensions 900 x 420 pixels. I tried doing some research online to see what the best dimensions were for creating a large header. I don’t need it to be the whole width of my site but I would like it to be fairly large. But when I upload it to my site, it shrinks down so that it’s miniscule. I tried changing the header height and width dimensions on my theme’s CSS to be the same dimensions (900 x 420 px) but that didn’t seem to do anything to the size of my header after I tried refreshing and reuploading the image. I tried making my image larger but no matter how big the image is, it remains the same small size on my site. Does anyone know how to make the header image larger?