you need to decide on what sensors and actuators you need.
if you are using modified servos to drive the vehicle, then you need 1 digital pin per servo.
motors usually need 2 digital pins each
photoresistors / ir detectors / rangefinders etc... need 1 analog pin each
a lot of sonars need a 2 wire i2c bus
Without knowing what you are planning on using, nobody can tell you the answer.
An mcu with less pins is usually smaller in size and quite often easier to learn but usually come with less features
an mcu with a pin count of around 40 is bigger, has more features, has more pins but will take a lot more learning because of the features.
an mcu with more than 40 pins is normally surface mount and therefore smaller than the above 2 but not recommended for the beginner because they are extremely difficult to hand solder.
Take a look at the microcontroller board of the $50 robot and see if it might be what you need (click on the society of robots logo at the top of the page)