The logo size will be limited by the container it sits in. Uploading a bigger logo will not automatically change the header shape or size, so it will be constrained by the header container in which the logo image sits.
Look in Journal > Skins > Edit > Header > and Quick-Edit the active header module. Once the module popup opens look in General > Header Height. This is the place where you can increase the header height, which in turn increases the logo area as well.
Additionally, you can add a container width as well as spacing around the logo image in the same header module in the Logo > Padding option. Remove any padding if already present to enlarge the logo.
More info on the logo here: https://docs.journal-theme.com/docs/skins/logo