By default this theme only allows you to have a maximum logo size of 250px. However your logo could look really small at this size. So to increase it you can edit the theme code to set a new maximum upper limit. This is done by going to the code edit section and editing the header.liquid file.
Find the following
{
"type": "range",
"id": "logo_width",
"min": 50,
"max": 250,
"step": 10,
"default": 100,
"unit": "t:sections.header.settings.logo_width.unit",
"label": "t:sections.header.settings.logo_width.label"
},and change this to
{
"type": "range",
"id": "logo_width",
"min": 50,
"max": 400,
"step": 10,
"default": 100,
"unit": "t:sections.header.settings.logo_width.unit",
"label": "t:sections.header.settings.logo_width.label"
},