I've read the Q and A here about safety of metro to get to the city center from the airport in Mexico City. I have read some more recent conflicting information that says that anything larger than a briefcase or back-pack will not be allowed on the metro. Can anyone confirm or deny that? I have a carry on roller bag that I typically use.
A large suitcase is usually acceptable. (Although at times - there may simply be no space)
There should be no real problems in terms of your bag being allowed, but it is recommended you do not take luggage.
There are endless stories of pickpockets, stealing etc, so bear in mind that any luggage you take is at your own risk. For a safer option, I would recommend taking a cab and following the tips below.
The two most common recommendations for a safe cab riding experience are to make sure you take an official cab, and to notify a person you trust of the license plate number of the cab you are riding. There is a free app available for iPhone, Android, and Blackberry (soon) that allows you to verify if a cab is official by comparing the taxi license plate number with the government provided data and that lets you communicate through Facebook, Twitter and/or email the license plate number of the cab you have taken or even communicate an emergency through these mediums. The free service is called Taxiaviso
Protect your personal information. There are pickpockets in Mexico City. Purses and bulky, full pockets are quite attractive. Do not keep your passports, money, identification, and other important items hanging out for someone to steal. Place items in a hotel safe with a proper locking mechanism, or tuck them away inside your clothes. The "Metro" subway system can get extremely crowded, which creates opportunities for pickpockets on cars that are often standing room only. Watch out for small groups of "interesting" people playing "magic" tricks near the entrances to Metro stations as these can be a ruse to have tourists gather round while others in the "troupe", acting as audience members, bump and push for a view of the "magicians" but in fact may be reaching into your bags or pockets.
Do not show money in front of others as this generally attracts pickpockets. Be vigilant when using ATM machines, be sure to hide your money safely away before leaving the ATM booth. Use ATM's inside a secured place such as in a bank. In crowded public places such as the North and South long distance bus terminals, be sure nobody is following after you after you've withdrawn money from the ATM.