Blogger or a Website owner should always see to it that their blog/website is user friendly (includes easy navigation,simple language,a good design) and also an important part of it is compatibility in different screen resolutions.The most used resolutions are 1200 x 800 and 1024 x 768. If your blog/website is not compatible with all major screen resolutions your blog/website will look really bad,and it will be bad impression.
An Online tool called viewlike.us actually help you dealing with this issue.
All you have to do is enter the URL of your blog/website.You can check on all screen resolutions including the iphone.
Check which is the most commonly used screen resolution