That depends on how many video's you get submitted, and the size of them. I would suggest more space than that though, as that will not let your site grow very big before you need to increase your specs.
How large is the average video? Divide 25GB by that and you'll get an idea of the maximum capacity. For those who don't think 25GB is large, consider this. I've just checked the monitoring for a group of 40 shared hosting servers that have a whole mixture of sites. Total website diskspace used - 800GB - and that is for 1000's of sites but doesn't include logs or OS, or databases. I'll stress this is shared hosting only - no reseller accounts, VPSs, or dedicated servers included. We're in the process of consolidating and capacity planning and I was suprised how little space is actually being used. We bring on new servers for performance reasons, not when the disks are full, so it seems like a lot of servers for so little diskspace used. I've just looked at one of the MySQL DB servers - 376 databases, 54GB of space being used. The average site doesn't use much diskspace.
I totally agree that the average site does not use the amount of space allotted to it. This is a marketing technique, sell someone 100gb of space, when the site they have may never exceed 100mb of space. This is how overselling works, on the good chance that users will never get close to the usage allowed. The site in question though is a video upload site, not one of your average sites. Though it may start off small, it could grow quickly, thus my recommendations.
Go For Unlimited...! Or you can go for 25Gb till it ends up and can update or upgrade your plan to unlimited once it gets finished..! 25Gb would be enough if your site gets average visitors..!