DC motors in series to get 5 Volts?
What you need to understand first is 

How DC motors work

Most likely you have a brushed DC motor. It will have an equivalent circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

When you use a motor as a generator,\$V_{emf}\$ will increase with the speed. However, V_mot that you can measure on motor's terminals is \$V_{emf} - R_{mot}*I\$. When you measured voltage on an open circuit, there was (almost) no current flowing, so you have measured pure \$V_{emf}\$. When you connect the load, there is some current, that causes voltage drop on internal motor resistance, and measured voltage is smaller.

How to find out what motor you need?

Assuming you want to use just a single motor:


Find out maximum current that your charger will draw, and a voltage that it will require. Keep in mind that some batteries (especially those found in cell phones) can require specific charging characteristic (constant current, voltage etc.).
Find a motor that will satisfy following condition: $$ V_{req} = V_{emf} - I_{req} * R_{mot} $$ where \$V_{req}\$ and \$I_{req}\$ are your required voltage and current.
You can assume that \$V_{emf}\$ will be equal to motor's rated voltage at no load speed. You can usually find these parameters in motor's datasheet. If there is no resistance given, you can calculate it by dividing rated voltage by stall (maximum) current of a motor.  


EDIT:

Series vs. parallel

If you want to have two motors, first make sure that they are both same type. That will not guarantee their specs will be exactly the same, but we want them to be as closely matched as possible. 


Series connection will give you higher voltage with the same current, but if you have 5V under the full load, when the current consumed by your charger/battery will go down, the output voltage will rise to 2 times the voltage of a single motor under no load. Make sure that your device can withstand this.
Parallel connection will allow for more current to be supplied. As I have already mentioned, it's the current flowing through a motor, that causes the voltage drop from higher voltage under no load to smaller with full load. Parallel connection of two motors will reduce the current flowing through a single motor by two times, thus reducing also the voltage drop. I would add a diode in series with each motor, to prevent current flowing in reverse through a motor if for some reason the voltages they generate differ.


Additional thoughts on your device


From what you have stated in your comment, you want to attach these motors to wheels of a suitcase. I disagree, that they will be guaranteed to have the same speed. Imagine a situation when you turn. Do the wheels travel the same distance then? 
You can push the suitcase backward, so the motors will also spin back, and generate reverse voltage. Are you prepared for this?
The voltage generated by the motor will vary with the speed with which it rotates. Are you ready to handle situations like walking slowly or running?