Find the largest N digit number in a W by H grid of digits
CJam, 39 36 35 34 bytes
qN/)i\[{zW%_}4*]ff{_,@e<ew:i}e_:e>
Just quickly, before @Dennis wakes up :P
Try it online.
Explanation
The basic algorithm is to take all four rotations of the grid and split each row into chunks of length N
(or the row length, whichever's smaller). Then convert the chunks to ints and take the largest.
qN/ Split input by newlines, giving an array of lines
)i\ Drop N from the array and put at bottom
[ ] Wrap in array...
{ }4* Perform 4 times...
zW%_ Rotate grid anticlockwise and push a copy
Note that this gives an array of 5 grids [CCW1 CCW2 CCW3 CCW4 CCW4]
ff{ } For each grid row, mapping with N as an extra parameter...
_, Push length of row
@e< Take min with N
ew Split into chunks
:i Convert to ints
e_ Flatten that array
:e> Take cumulative max