Not unreasonable to be excited about buying Yaktrax. They are the most amazing things ever! I was very pregnant last winter and hate walking on snow and ice. I only fell over when I forgot to put them on.
There should be a size guide on their website...maybe under FAQs? If you want them to go over walking boots etc you will probably need to go for a larger size as the sizes are not terribly generous. Having said that, mine still work fine on my trainers.