HTML IFrame alignment

Site URL: http://www.whereslewisat.com/walking-the-wainwrights/

Hello! I have embedded a map on my site and I was wanting to make it bigger, however when I add it to my site I cannot control the alignment of it, it just goes from left alightment over to the right?

What code would I need to add to the HTML coding, in order to make it appear centre to the page?

This is the code that Google MyMaps has given me to add to the site through the Squarespace coding widget, it doesnt appear to feature any alightment detail in the code?

This page is on my site at www.whereslewisat.com/walking-the-wainwrights

<iframe src="https://www.google.com/maps/d/embed?mid=1-YyaFsxihH4IsTFqAyhC29KHO2m51pfC&ehbc=2E312F" width="1500" height="800"></iframe>

Also I am just asking to see if this is possible.. is there a code that could be added to make the HTML page-load to the exact width of the site page by default as well? This map is the main part of the page so would like it as big as possible, if I can make it 1500x800 and centre then great, however if there is a way to make it so it is max-width that would be great too!