Using more than 8 digits using the sevseg library
You can change the pre-defined maximum number of digits at the top of the SevSeg.h header file to as many as 10, if you use long or unsigned long when you call setNumber.  It is defined here, on line 27:
#define MAXNUMDIGITS 8 //Increase this number to support larger displays

This the version referenced from the Arduino Playground SevSeg.  Be sure to use the same number in your modulo, too:
void loop()
{
  //local vars
  static byte decPlace = 0;

  sevseg.setNumber(8888,decPlace);
  decPlace++;
  decPlace %= 10; //rollover back to 0 once variable gets to 10

decPlaces is mostly useful for floating-point numbers: it specifies how many digits after the decimal point should be displayed.  For integer values, it specifies how many leading zeroes will be displayed.
However (as you discovered), you cannot call setNumber with anything larger than a long, so you are limited to 10 or fewer digits for MAXNUMDIGITS.  (There is no byte overflow -- it's just limited to the number of decimal digits in 232-1.)  This is a limit of one SevSeg instance.  You could have two instances of SevSeg like this:
#include &lt;SevSeg.h&gt;
SevSeg sevseg1;
SevSeg sevseg2;

void setup() 
{
  byte segmentPins[] = {6, 7, 8, 9, 10, 11, 12, 13}; //Segments: A,B,C,D,E,F,G,Period

  byte digitPins1[] = {2, 3, 4, 5, 22, 23, 24};
  sevseg1.begin(COMMON_ANODE, sizeof(digitPins1), digitPins1, segmentPins);
  sevseg1.setBrightness(10);
  
  byte digitPins2[] = {14,15,16,17,18,19,20,21};
  sevseg2.begin(COMMON_ANODE, sizeof(digitPins2), digitPins2, segmentPins);
  sevseg2.setBrightness(10);
  
  sevseg1.setNumber(  7654321UL, 7 );
  sevseg2.setNumber( 87654321UL, 8 );
}

void loop() 
{ 
  sevseg1.refreshDisplay();
  sevseg2.refreshDisplay();
}

The first instance handles 7 digits, and the second instances handles 8 digits.  Note the use of &quot;UL&quot; to force and unsigned long call.
Update 11-DEC-2015

Using the following code, I can't get the 3rd set of 5 numbers to display. I have changed the sevseg.h file to 15, but that shouldn't matter:

First, the MAXNUMDIGITS is the maximum per instance, so you could set it to 5 to save some precious RAM.
Second, you have a copy and paste error:
sevseg1.refreshDisplay();
sevseg2.refreshDisplay();
sevseg2.refreshDisplay();   &lt;--  Oopsies!  :)

Third, I assume the setNumber calls are placeholders for what you're really going to use.  Since they always set the same value, they could be in setup.  In your final program, there must be some input that will be used to calculate a number to display with setNumber.