Apologies

I just wanted to write a short post to apologise to people who do read my blog. As I am blogging from an iMac, all the fonts and font sizes look alright to me. However, I noticed that when I am reading it from a PC, the blog posts come in all different fonts and sizes! I'm not sure how I can correct that so my apologies in advance if the posts are hard to read.

If anyone can tell me how to fix this problem, I would be grateful. :))