HOW BIG SHOULD MY SIGN BE?

This answer to this question is critically important, because in this context, size definitely matters. 
There's more than one answeer, so we'll address it in several ways.
In this segment, we'll cover the "ideal" situation.
The "ideal" situation is one in which your sign is faced with NO visual clutter, meaning no competing signs.

Most importantly, your sign should be sized appropriately to the DISTANCE from which you want people to see it.
Below is a chart developed by the California Institute of Technology.
It describes, for the height of each letter or number, its maximum readable distance, and the readable distance for maximum impact.

LETTER      MAXIMUM          READABLE
HEIGHT      READABLE         DISTANCE FOR
                DISTANCE          MAXIMUM IMPACT

3 inches      100 feet             30 feet
4 inches      150 feet             40 feet
6 inches      200 feet             60 feet
8 inches      350 feet             80 feet
9 inches      400 feet             90 feet
10 inches     450 feet            100 feet
12 inches     525 feet            120 feet
15 inches     630 feet            150 feet
18 inches     750 feet            180 feet
24 inches     1,000 feet         240 feet
30 inches     1,250 feet         300 feet
36 inches     1,500 feet         360 feet
42 inches     1,750 feet         420 feet
48 inches     2,000 feet         480 feet
54 inches     2,250 feet         540 feet
60 inches     2,500 feet         600 feet

So, if you want your sign to be visible from a distance of 600 feet, your letters must be at least 15 inches tall JUST TO BE VISIBLE in an UNCLUTTERED visual environment.  And for the sake of simplicity, we calculate that for every character that is 15" tall, that character with its appropriate spacing is also 15" wide.  Margins at top, bottom and between lines should be at least half the character height.

The next step:  Your message.  How long is it?  The shorter, the better.  Whenever possible, eight words or less is preferred.

Let's say your message is:  FOR LEASE 12,000 SQ. FT. WITH DOCK JASON 800-123-4567  and you want the entire message to be seen from 600 feet.  
How big should the sign be?

For communications clarity, we'd stack the message this way:

FOR LEASE
12,000 SQ. FT. WITH DOCK
JASON 800-123-4567

The height dimension would be: 7.5+15+7.5+15+7.5+15+7.5 = 75 inches = 6.25 feet
The width dimension is determined by the longest line, or 17 characters x 15 inches = 255 inches = 21.25 feet.

Of course, adjustments can be made with font, colors, etc., but this provides a ballpark idea or formula for how big your sign should be.