You don't have to work out how big it would have to be. According to Genesis chpter 6, the ark was made of gopher wood and was 300 cubits long by 50 cubits wide by 30 cubits deep.
Since a cubit is 17.6 inches or about 45cm. this gives you a total of around 40,000 cubic metres, assuming a rather square and un-hydrodynamic arc. Alternatively, you've got three floors, totalling around 9000 square metres of floor space, plus a roof if your animals don't mind getting wet.
Somehow, you have to fit 14 of each clean and 2 of each unclean animal.
Are elephants clean or not? Anybody know? How about hippos and rhinos?
Of course you have to travel round the world collecting them in the first place. Those polar bears are quite tricky to get, sailing from the middle east to the arctic in a gopher wood ocean liner with no engine. Unless pandas, kangaroos, penguins and the like were all living in and around the middle east at this point. Of course, if you argue that this was the case then you have to accept that Noah must have sailed round dropping them all off on different continents afterwards.
The problems with the ark are just beginning. After all the creative packing and circumnavigation you've done, you need to work 24 hour shifts feeding at one end and shovelling at the other for every single animal.
Assuming you haven't been mauled by any of the vicious ones, trampled by one of the heavy ones or suffered a nervous breakdown by this point.
And while we're on the subject, just how hard would it have to rain to cover all land masses on the planet in 40 days? And where did all the water go?
And why do I care anyway?
I posted in this forum to see if some would try to make some calculations on it. Just for fun. 
) and has the rest transported in egg form. Pity the cryo-sleep chamber wasn't en vogue back then...
