To my knowledge, PCM is especially challenging for neurosurgeons. Three cm is often a limit that a PCM can or can not engulf large vessels or cranial nerves. If it is larger than 3 cm, it often involves some skullbase structure that is very important. Of course, 3 cm is just a common limit, not for all PCMs.

Thanks Bo Qiu for pointing out that 3 cm is a general limit for meningiomas and Stephanie E. Weiss for discussing deep into tumor biology. 

Just to chip in few other numbers:

Pituitary >1 cm macro-adenoma >2.5 giant pituitary tumor

aneurysms >2.5 cm giant

But the problem with the available statistics is present in many sub-specialties, for example Role of ICP monitoring in Neuro ITU is not LEVEL 1 evidence. At best they are Level 2. 

petroclival meningioma tumor size is classified as follows : small (4,5 cm.) using maximal tumor diameter. A petroclival meningioma can only be defined as such if the tumor originates medial to the cranial nerves V to XI. Thus a meningioma lateral to the IAC should not be considered petroclival. Large PCM's are stretching the cranial nerves posteriorly which makes them often vulnerable to manipulate. But even more than the volume the consistency and relation to the surrounding arachnoidea determines the resection feasibility. 

Prof Almefty uses another classification and the size is classified 4 cm..obviously other paramenters are important!

http://thejns.org/doi/abs/10.3171/2013.8.JNS13535

TO TOUCH UPON FEW POINTS IN::

SRS+LARGE BENIGN SKULL BASE MENINGIOMAS 

The traditional optimal radiosurgery treatment size is less than 3 cm in diameter, but this is arbitrary. Symptomatic tumors and those larger than 3 cm should be treated by resection and then by radiosurgery targeting areas of residual tumor.

A larger treatment volume requires a lower dose to prevent undue risk of radiation necrosis. Treatment parameters that measure dose–volume relationships such as the 10-Gy volume are important to understand because they provide a benchmark for determining radiation risks.[4] A suggested prescription dose at the tumor margin for meningiomas is 18 Gy (< 1 cm), 16 Gy (1–3 cm), and 12 to 14 Gy (> 3 cm).

The brain-stem can tolerate up to 15 Gy, but facial nerve injury and decreased hearing can result with doses in this range when applied to acoustic tumors.[8] Lowering the dose to 12 Gy in the treatment of acoustic neuromas has reduced the incidence of facial nerve injury to less than 5%.[25] Depending on the volume, meningiomas along the skull base in the posterior fossa should receive a maximum of 12 to 16 Gy.

Petroclival region meningiomas are challenging to treat surgically because they are often intimately associated with multiple lower cranial nerves and the brainstem and because their extensive dural attachments along the skull base make complete resection impossible. Subach, et al.,[33] retrospectively analyzed 62 cases treated with GKS, of which 39 patients (63%) had undergone at least one prior resection. The mean tumor volume was 13.7 cm[3], and the mean tumor margin dose was 15 Gy. Neurological improvement was seen in 21%, whereas 13% worsened. Tumor volumes decreased in 23%, remained stable in 68%, and increased in 8%. New cranial nerve deficits developed within 24 months of GKS in five patients (8%), but resolved completely in two.

Because petroclival meningiomas tend to present as larger tumors, surgical debulking plays a much greater role in their treatment than in cavernous sinus meningiomas. Small tumors, including those causing neurological deficit, may be appropriately treated with SRS alone because an improvement in symptoms can be seen. Large tumors should be surgically debulked first, and radiosurgery undertaken postoperatively for remnants. Location of the seventh to eighth cranial nerve complex should be identified so that the radiation dose in this area does not exceed 12 Gy.

http://www.medscape.com/viewarticle/456130_4

http://www.ro-journal.com/content/6/1/36

>3cm is commonly associated with involvement of the 6th. nerve and tumor infiltration to the clival cavernous sinus and crossing the midline making tumor resection more difficult.