Estimations can be quite tricky to do and even trickier to get from an developer. One way to go around that is to play planning poker. Follow that link for some great explanations by the SCRUM-guru Henrik Kniberg.
And as the Internet is a fantastic thing someone of course as made an online version, that can be found here http://www.planningpoker.com/
I think that it’s more important to get together than to do the actual poker playing, but if you don’t have that option this will have to do.