As a part of lexical knowledge, the present study attempted to develop an English collocation proficiency test to measure the size of second language learners` collocation knowledge. The existing collocation tests, such as DET (Eyckmans, Boers, & Demecheleer, 2004), CONTRIX (Revier, 2009), and COLLEX/COLLMATCH (Gyllstad, 2009), were developed as achievement tests or could only test some specific types of collocations (e.g. Verb + Noun collocations). To address this deficiency, a new Collocation Size Test (CST) was developed using the recently constructed General Service Collocations (GSC) (Shin, Chon, Lee, & Park, 2017) which was first extracted from the large-scale Corpus of Contemporary American English (COCA) and then refined by comparing 8 representative reference corpora, such as Freiburg-Brown Corpus, Freiburg-LOB Corpus, and Australian Corpus of English. The test of so-called GSC CST is based on the first 10 levels of GSC with 10 items at each level. Five of the items measured meaning recognition while the others assessed the learners` ability to recognize the correct form of collocations. Each item could be considered as representing knowledge of 50 collocations within the collocation band. In the validation process of GSC CST, the test showed a reliability of .91.