What kind/size of rehydration bladder?

Hi, I bought a Karrimor Rapid 10 a couple of years ago for cycling which comes with the option for a rehydration bladder. I didn't get one at the time, but am considering one for my Amsterdam ride, as it will be easier than water bottles and I don't mind stuff on my back. Problem is, the product appears to be discontinued, does anybody know what size bladder I would need? Also, what do I need to look out for when buying one?

I bought a Karrimor Wind backpack last year from my local sports direct store http://www.sportsdirect.com . There was a lonely bladder hanging up near it (about 2ltrs). Pocket internally for bladder option and hole/straps for tube. Had to get second bladder off ebay as store only had one and PieApprentice wanted one as well. A search for 'bladder' will bring up a few.
Was in the store yesterday. They had the backpacks in, didn't notice any bladders though, sure they were £13.99 not £14.99 as per website. Can't find the bladders on the website either but were only about £5
Any idea what size the pocket is? Can't see them being vastly different. I'll even measure mine up if you want.
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