Sentence:  For four of the six ( the anaconda and the amethystine python cannot be included for lack of data ) there is also a correlation between <WSD>size</WSD> at maturity and maximum length , the boa constrictor being the smallest and the Indian python the next in size at the former stage . 
Meaning of the word marked with <WSD> tags:  the physical magnitude of something (how big it is)