x2 on all the above replies.
At this point, size doesn't mean much; it must be evaluated with many other tests.
WHO are you seeing?
Probably a GI doc, who you would have seen for rectal bleeding.
You need to get an Oncologist on board, and an Oncology Surgeon; both from within your insurance's network, and both of whom you interview and feel comfortable with.
And ideally, all the docs should be within the same hospital, one which has a very good cancer center.
Tumors are generally round and flat, thus the name 'cancer' (from the greek ofr 'crab', being somewhat crab-shaped: round and flat with tendrils that spread out).
But some are solid, round/shperoid, others are oblong, etc.
So the one dimension, 5 cm, doesn't give a very good idea of its overall shape.
5 cm = 2 inches.
Yours could be a 2-inch sphere, the size of a billiard ball.
Or 2-inches round, but only 1-cm thick (3/8").
Or any possible configuration/shape in between.
Please don't agonize any further about what the size might mean.
If it has spread, that will be determined with further testing.
Bottom line is that it is IMMINENTLY TREATABLE, with chemo, radiation, and surgery.
You will need several tests during the next few weeks, including colonoscopy (is that what you've had thus far?). possible laproscopic biopsy/surgery, blood tests, and CT scans and possibly PET scan.
See our 'Terminolgy' thread to familiarize yourself with terminology.
Spend your internet time learning a little bit about those tests and procedures.
Check with your insurance company relative to copays (in and out of network), and other expenses.
You will probably have out-of-pocket totals THIS YEAR (now thru 12/31/11) that 'max-out' at the $5,000 or so typical maximum patient cost, so get your finances in order.
That maximum annual will re-commence 1/1/12.
Sorry you qualify to join our club, but you've come to the right place.
There are also other websites, especially Colon Cancer Alliance and American Cancer Society.
M 61, Dx 6/10 CRC st 4, unknown primary CEA 843
2 kg peritoneal mass, met to skull; no surgery
Various regimens of all CRC chemo drugs
Mets to lung 8/11 CEA 135
Folfiri/Erb/Ava 12/11 CEA 320
No progression 5/12 CEA 192