I don't know whether you're talking about the size of the number keys on the phone or the number size displayed on the phone screen but a Samsung mobile displays the number pretty big.
As for key size then I'm not sure if they're around anymore as most mobiles tend to go for tiny keys as they're the trend. But I could be wrong.
Take a look on this website :
http://www.mobile-phones-uk.org.uk/
Not only do they review mobiles but they'll show pictures of the mobiles, so you can take a look for a mobile with suitable size keys and read the review about it.