I would like to get two hamsters and I would like to know how big the cage needs to be so they have plenty of space to eat,sleep,and play.
First: Depending on the species of hamster you will keep, you may need to limit yourself to one; many species are actually solitary and you will only keep fighting if keeping them together.
Now, the cage: Ideally, we are talking about 1m² ground floor, that's roughly 10ft², for one hamster. As there are few cages supporting that, so you may need to build yourself. Luckily, that is relatively easy AND usually cheaper than a decent bought cage.
Basic setup for a hamster-cage may look like this:
A wooden box, one side open. Make sure the wood is coated against urine soaking in. (Thanks Christy B.)
Acrylic glass on one side (just buy grooved strips to hold the acrylic. You can then remove the acrylic for cleaning the cage, too!)
Add a lid with hamster-proof wire.
Viola, you are all set up to keep a Hamster. Cost of this will vary depending on where you live, but considering most Hamster-cages are either too small or really expansive (or both....) it's worth the effort to make your Hamster happy!
As a building-base, you can also use a simple Cupboard, just make sure the backside is not one of the flimsy plywood-backs.
If building yourself is not an option, look into decent-sized aquariums. They work, too, but are a little harder to clean (no front-side to open up). Sadly... the moment Stores write "Hamster-cage" on something, 90% of the time it's NOT suitable for a Hamster due to size.