A wheelie drop, also known as a mono, is when you pedal while having your front wheel in the air. A manual is when you don't pedal. Mono's are easier as their is constant power going to the rear wheel making it easier to pull up and keep up.

Also when dropping doing a two foot drop is basically the same as a six foot drop. This is only in style however, they will feel totally different as you gain speed while falling. Just practice, this is the only way to get better. Do you think Grant Allen or Wade simmons just woke up one morning before they started riding and said, "lets go do a 30 foot drop." NO! They built up to these.