If you are in the market for a Viking helmet or shield to complete your ensemble, there are a number of practical considerations that go into the decision. From deciding on how historically accurate to be, to what weather you will be using it in, the only major difference between you and a historical Viking is that you will hopefully not have to depend on your shield for your life. In looking for a shield, there are essentially four major considerations.
You are going to have a round shield. Although some incredibly spare references suggest that later Vikings might have occasionally used tear drop shields, no archaeological evidence supports this. Exactly how large your round shield should be is actually up for some debate: too small a shield and you will be open to attack, too large and you will hamper your own movement and perception. Generally, the size you want should go about from your neck to your knees in a slightly crouched “fighting” position. They are not one size fits all, but, again, different Vikings might have preferred different sizes to suit their fighting styles.
You are going to have to carry this thing wherever you go in costume. You might have a good idea, when looking at a Viking helmet for sale, about how much weight you want on your head. When looking for a shield, you need to consider how much you want to hold up in one hand in whatever situation you will be in. Viking shields are made of wood, and some were likely bound in leather. Either can be quite weighty, especially depending on what kind of wood it is. Add water weight to that if you choose to get one that is not sealed against the elements.
If you are going to be in a mock battle, your shield might take more than mock damage. Make sure that you choose the shield appropriate to the use to which you will put it. If you only need it complete your look, how many ax strikes it can take might be a moot point. However, if you need it to take punishment, make sure you get one that is sturdily constructed and reinforced.
Finally, you will need to decide what colors and designs you will have on your shield. Although there are a limited number of historical colors to choose from (evidence exists for the combinations of black/yellow and red/white), this is one area where you might let your individuality or Viking group (if applicable) help you decide. There is not a lot of surviving evidence to suggest that any design was off the table in terms of decoration, but remember that anything added to the shield adds more weight, even paint. This final consideration helps ensure that when you or your Vikings shield themselves, they will look both accurate and intimidating.