The reason it's somewhat confusing is that nearly every organ system, if not organ, has its own staging system. Thus, for some tumors 4cm is large (like a T3) but for others that might be a T1. Complicating it more, sometimes size is not the issue, it is extent of invasion (colon cancers, for example, have the T stage defined on how far the cancer invades - through the muscle, through the fat, etc). Complicating it even further is that some systems, like GYN, subclassify T stage beyond just 1,2,3. There are T1a tumors, T3c tumors, etc.
Basically, a T1 tumor is generally considered fairly localized and non-invasive. T4 tumors are widely invasive (esophageal cancer directly invading a lung, for example).
N refers to nodal metastasis, which again depends on the system you are in. N1 nodes are usually close to the organ, N2 nodes further. Some systems go beyond this depending on how much they want to classify it.
M is usually defined as either M0 or M1 (or Mx, which is "unknown"). There may be some cancers which go up to M2 if they want to define a sub-diaphragmatic met vs a super-diaphragmatic met, for example.
Basically, if you are interested enough you have to learn separate staging criteria for each.
Here is Renal Cell Carcinoma, for example:
Primary tumor (T):
TX: Primary tumor cannot be assessed
T0: No evidence of primary tumor
T1: Tumor 7 cm or less, limited to kidney
T2: Tumor greater than 7 cm, limited to kidney
T3: Tumor extends into major veins/adrenal/ perinephric tissue; not beyond Gerota's fascia
T3a: Tumor invades adrenal/perinephric fat
T3b: Tumor extends into renal vein(s) or vena cava below diaphragm
T3c: Tumor extends into vena cava above diaphragm
T4: Tumor invades beyond Gerota's fascia
N - Regional lymph nodes
NX: Regional nodes cannot be assessed
N0: No regional lymph node metastasis
N1: Metastasis in a single regional lymph node
N2: Metastasis in more than one regional lymph node
M - Distant metastasis
MX: Distant metastasis cannot be assessed
M0: No distant metastasis
M1: Distant metastasis
Note that with this, you can have a tumor smaller than 7cm (T1) but if it fits the right criteria, it can possibly be a T4 due to its invasiveness.
Other cancers will be different. There is an AJCC staging book freely available. Some are very complicated (Endometrial CA, Prostate CA). This is a simple one!
Want more? Once the TNM staging is completed, the result is often fit into a further, overall stage, I-IV. In short, stage I is confined and small, stage IV is pretty bad.
