This should put you in the right direction:
string myString = "db 4Dh, 5Ah, 80h, 00h, 01h, 00h, 00h, 00h, 04h, 00h, 10h, 00h, 0FFh, 0FFh, 00h, db 4Dh, 5Ah, 80h, 00h, 01h, 00h, 00h, 00h, 04h, 00h, 10h, 00h, 0FFh, 0FFh, 00h";
StringBuilder sb = new StringBuilder();
string[] splitString = myString.Split(',');
for (int idx = 0; idx < splitString.Length; idx++)
{
sb.Append(splitString[idx] + ",");
if (idx > 0 && idx%15 == 0)
{
sb.Append('\n');
}
}
string output = sb.ToString();