Facebook Cover Photo 2018: width:height ratio is 16:9. 1920 pixels by 1080 pixels is a good size and will display nicely on all devices. Make sure that faces, text or important info is towards the middle of the image so that any cropping won't be noticed.
Louise M. has updated template sizes to help if you need more details.
While a developer can quickly get you up and running on a platform of your choosing, sometimes you want to try it out for yourself. Here's a good document about the pros and cons of each of the most popular website builders in 2018.
WordPress.org, Sitebuilder.com, Wix, BoldGrid, Shopify, Weebly, Squarespace, iPage Website Builder, GoDaddy Website Builder, WordPress.com, Webs.com