to which grammar

(1) WHICH 係指咩野???
"which" is a relative pronoun referring to the noun "degree".

(2) 點解要用 to which??? which 唔得???
查字典 (Longman):
degree = the level or amount of something (某事物的程度,水平或數量)
其中一個用法:
to a degree/to some degree/to a certain degree (到某一程度/在一定程度上)
e.g. We're all willing to support him to some degree.
e.g. I agree with you to a certain degree.

"degree" is the object of the prepositional phrase ("to a degree") in the following relative clause, so you have to put the preposition ("to" in this case) in front of the relative pronoun "which".
You can also use only "which" by putting the "to" at the end of the relative clause.
It is the degree which the size and purchasing power of a market segment can be measured to.

(3) 點可以把呢條relative clause 拆解為2個sentenses?
The above sentence uses defining relative clause (restrictive), so it can hardly split into two meaningful sentences.

(a) Measurability (= It) is a degree of something.
(b) The size and purchasing power of a market segment can be measured to a (certain) degree.

2011-11-10 09:56:07 補充：
so it can hardly be split into two meaningful sentences.
所以,難以分割成兩個有意義的句子。