Unlike many other types of footwear, these boots do not have the ability to stretch according to the shape of your feet. Thus, you need to purchase a pair of steel toe boots that is at least ½ a size larger than your normal shoe size. This will give your feet, especially your toes, extra room, in case they swell up during or after a hard day at work.
The best time to buy a pair of steel toe boots is in the afternoon; at this particular time of the day, your feet are at their largest, and you are sure to buy a comfortable size.
If you are planning to buy a new pair of steel toe boots, get them well ahead of time, and break them in gradually. Initially, wear them for a few minutes at a time, then progress to an hour, followed by a few hours, and finally all day. This will allow your feet to steadily become used to these type of boots. If you buy steel toe boots and wear them right away, you may get blisters.
Wear the best, thickest hiking or working socks when wearing steel toe boots. These may cost you a little extra, but they will protect your feet from getting blisters, and will last you a lot longer too.
If nothing seems to make the steel toe boots comfortable for you, consider wearing inserts. However, make sure that you wear good thick socks with the inserts too.