I believe a base of 360 sq inches is a minimum. I'd never use a Habitrail for a Syrian hamster, multiple cages or not lol We have a bin cage, equivalent to a 30g.
Here's a neat little post I remembered reading and it put it into perspective for me... credit to Sonic Ham of hamsterhideout.com
HH's-- 360 square inches
OSPCA's, Humane Society's, Popular Pets Magazine: Hamsters (Hamsters for Dummies, the Hamster Handbook (by: Patricia Bartlett) and the National Hamster Council has similar minimums varying by no more than a few inches)-- 10 gallon/ 2 square feet
RSPCA's 75cm by 50 cm Now 75cm by 40cm, though images below depict their old minimum.
German Style 100cm by 40 cm
One square Metre (I believe this was a minimum decided for syrians in the study theFeldhamster has on her blog):
Hamsters: The Ultimate Pocket Pet by Virginia Parker Guidry's 19 square inch minimum
I also measured out the USDA's minimum cage standard. I didn't compare the sizes using hamster supplies but a quarter and a 2 shillings coin. The USDA stands for the United States Department of Agriculture and they apparently regulate large scale breeders in the US. More info here: USDA licensed breeder Better?. I don't know how old that is and if it's been changed but using the measurements given in that article I came up with these.
One: For a syrian over 10 weeks of age-- 15 square inches
Eight: For a syrian up to 5 weeks of age-- 10 square inches
Three: For a syrian 5 to 10 weeks of age-- 12.5 square inches
Four: Dwarf up to five weeks of age-- 5 square inches
Five: Dwarf 5 to 10 weeks-- 7.5 square inches
Six: Dwarf over 10 weeks-- 9 square inches
Seven: Nursing Syrians (mother and litter)-- 121 square inches
One, never use the crittertrails. Rodents can easily escape from them AND THEY ARE DEATH TRAPS. They have horrid ventilation and so many places they can chew through the plastic, slip through the bars and strangle themselves. I have known people who's hamsters have died in them and I am appalled they are still sold after the long lists of fatalities they cause. Go with min a 20 gal tank with a lid you can snap shut and a mesh lid. I would say paper bedding is the best because it has less dust and dust can irritate their sensitive little lungs. Also a block food like Oxbow is best. I feed oxbow to my rats and have looked at the hamster blend which my mice use to eat. If properly feed you shouldn't need to worry about obesity and keep sunflower seeds down to a low. Corn isn't too good for them either and hay is not approved to be honest. They can't properly digest it. Fresh veggies and fruits are good, they need fresh food in their diets. And NEVER give hamsters peanut butter, it can get stuck in their check pouches and then you have trouble. My friends got an infection from that. Also try to keep them alone, a lot of people make the mistake of keeping them together and unless you are an expert on hamsters it's never a good idea, especially first timers. A single hamster with plenty of attention will be just happy as can be. Chews are crucial, they need them because they are chewers. Also try to find an exotic animal vet who knows about hamsters. Like most other rodents they are susceptible to URI which are often times fatal. You want to know you also have a vet in case of emergencies. Avoid wheels that are mesh or bars. They can get their feet caught in them and I have seen feet ripped clean off from those things. My friend chinchilla lost a foot in a mesh wheel. Clean water is a must, change the water every day with fresher water and clean it out weekly so nothing starts growing in it. Baby bottle cleaners are your best friends. Hamsters and any rodent are also a big investment. Though they do not live long they can cost a pretty penny. Toys will have to be replaced often from damage but when all is done and said I can't say their is a better pet than a rodent. They are tiny but have such BIG personalities it's hard to think such a small creature could be so full of love and magic. The more you socialize them and play with them the more excited they are to be with you. I also know people who have leash trained hamsters but do not trust the hamster leashes, they are actually a bit dangerous. If your hamster is on the larger side a ferret harness however can be modified and is safer but it has to be a vest harness not a harness like a dog harness.