Thank you for your fast response!
It is an art project so visual aspect is quite important. I was thinking in using quite big digits (3-4 inch). I do not know if this has a mayor impact on the structure of the electronic device.
Everything that makes things easier is welcome 
So if it is better I would go for the 32 digits.
As I mentioned before I am completely new to all this so I appreciate your patience and any advice to keep it as simple as possible.
I found this one: http://www.ebay.es/itm/Protoboard-700-contactos-Breadboard-placa-prototipos-soldadura-Arduino-/170927079955?pt=LH_DefaultDomain_186&hash=item27cc0c4213#ht_2473wt_1190 , is this big enough?
The digit changes every second. That is why it will take so long.
I already know how to make a simple 1 digit countdown, but I can not find the way to make the 25 digits one… any help with diagrams or code is more than welcome 