calculate the crossectional area of the downspout and use roughly the same crossection of pipe and should be OK

The formula is ...

LARGE DIAMETER SQUARED divided by SMALL DIAMETER SQUARED.

Example... Let us compare a 4" pipe to a 3" pipe

(4 x 4) / (3 x 3)= 16 / 9 = 1.77

This means a 4" pipe has the equivalent volume of 1.77 pipes that are 3".

We must also remember that the velocity of flow in a vertical pipe (down spout) is much greater than the velocity of flow in a horizontal pipe (horizontal storm drain).

The actual volume of a horizontal storm drain would be effected by such factors as the length of run, the pitch of the pipe and the "friction head factor" of the pipe material selected.

A basic rule of thumb is that a 4" horizontal drain line with a minimum of 1/4" per foot pitch can handle the same volume as a 3" downspout thus combining two 4" pipes into one pipe would require a 6" pipe.

(6x6)/(4x4) = 36/16 = 2.25

Thus a 6" pipe has the equivalent capacity of two 4" pipes or two 3" downspouts.

MATERIAL CHOICES:

CORRUGATED POLY PIPE: the advantages of corrugated poly pipe are:
1. Inexpensive- typically a 75' roll of 4" roll poly is about $35 to $40. Check prices carefully, you may find that running two 4" side by side would be cheaper than running one 6".
2. User friendly- corrugated roll pipe comes in 50', 75' and 100' rolls. This means you can usually run the entire run without joints. When joints are necessary they have connectors that just slip on the pipe and require no gluing.
3. Uneven terraine- being flexible roll pipe is more suitable to uneven terraine conditions.

PVC "SDR PIPE" If you look in your hardware or home supply center you will find a thinwall PVC pipe which is labelled on the pipe as SDR 20, SDR 30 or SDR35. SDR indicates the pipe is made to the "Size Dimension Ratio" standard. Under the SDR standard the actual thickness of the pipe wall is determined by dividing the pipe diameter by the index number thus a 4" SDR-20 pipe would have a wall thickness of 4"/20= 0.2". SDR pipe may not be used for DWV (Drain, waste & vent) applications but it is approved for storm drainage. SDR pipe may be white like sched 40PVC or it may also be light green or light blue in color and usually it will have a preformed end bell on one end of the length to permit joining. It may be glued but glueing is not required for storm drains. SDR pipe has an advantage on long runs because it has a much lower "friction head factor" than corrugated roll pipe.

SCHEDULE 40 PVC: Schedule 40 PVC is code approved and it would be perfectly suitable although it would generally be considered overkill and it would be the most expensive.

Use 4" or 6" main to the street. If you plan to run over it with equipment, cars, etc, use sch 40, otherwise, use sch20 sewer pipe as mentioned above.
Use 4" corrugated to tie each downspout into the 4" main if you want. I would avoid it if possible, but it has its place.
Do not glue it. You do not need to, and once its in, you may decide to add drains. Nothing more tempting than adding drains to a system you know is there.

The problem is usually trying to calculate the volume of condensate.
It is usually a rather small amount, thus the reason you can get away with small lines and small condensate pumps for each unit.

Each condensate pump has a small holding tank to accumulate enough condensate the pump can run for a few minutes the volume is so low.

Most of the time it is just a few ounces per hour.
Condensate is cold distilled water and tends to be aggressive.
Plastic lines are a good idea.