is a 6 years Biology Degree comparable to a Master's Degree?
You are in a slightly tight but not an impossible to overcome situation.

What you describe relates very closely to the pre-Bologna_Process situation in Europe. People especially in Science- and Engineering-related disciplines had 5 to 6 years Bachelors (usually called Diplomas). When these graduates applied for graduate studies, especially in UK where the BSc in usually 3 years, they found themselves massively downplaying their qualifications. (The situation is somewhat sorted now.) Currently the old diplomas are almost universally accepted as Masters degrees because the retroactively calculation of ECTS points puts them nearly always in the same range as a Masters degree. 

When you say abroad you do not specify where. 

If abroad translates to Europe then things are quite straightforward. You say you have a Masters and leave things be. Departments are quite used to the phenomenon of "5/6 years of studies". Most of admissions committees will not blink an eye. It would almost certainty good to present it in your CV as "Integrated BSc and MSc studies" or something equivalent, so one can immediately spot it. Your programme of studies is far from unheard in Europe.

If abroad translates to US/Canada I do not have personal experiences; I think it will greatly dependent to the particular university's guideline. (I will be happy to be corrected at this point from a North American native if there is some standardised framework I do not know of). I can tell you for sure that 4-year BSc are not counted as Masters. I know very few people who applied for PhD straight out of a 5-year Engineering degrees from Greece; both them were accepted but they were very strong students anyway so I would not extrapolate based on them. Some others went for Masters (and continued to PhD eventually).

In general, a 6-years degree puts you in a very strong position if you are to be compared with simple BSc holder for PhD admission, probably your compare favourably to most MSci programmes as well. I would not worry much in that aspect. Having strong grades in those six years will be more important than what one equates your certificate to. (Assuming your references, cover letter, etc. are the same.)