If you are using Shopify's Dawn theme, the store locator is probably displaying very small. This is because Dawn's default page layout is set to narrow. This is what you can do to make it look wider and overall much better.
Go to your Shopify themes page
Click ... -> Edit Code
Select the file named main-page.liquid and around line 21, you will find something like this:
<div class="page-width page-width--narrow section-{{ section.id }}-padding">
Change the word narrow to wide so it looks like this:
<div class="page-width page-width--wide section-{{ section.id }}-padding">
Then click Save at the top right.
And that's it! The store locator should now look wider.