I've converted an avi file to dvd format so I can burn it and play it on a dvd player with menus and all that, my question is (might be stupid) is a 4 GB dvd disc enough to burn the avi file, it's like a 1.5 GB after conversion. I'm asking because I remember facing some troubles burning a dvd long time ago.
You really shouldn't have a problem.
Are you using Windows 7? I had to convert all of my movies beforehand when I wanted them on DVD before I started using Windows 7, but the DVD Maker program that comes with 7 is fantastic and does it all for you - I just have a regular DVD player (~10 years old) and every DVD I've made with it works perfectly.
It depends, since a DVD5 (4.7 GB) supports 120 minutes of video. So if your 1.5 GB avi file has more than 120 minutes of video, you won't be able to write it on a DVD5.
A DVD5 yes supports 120 mins of video if encoded to DVD standard, but you can put a low bitrate divx movie on a DVD that can be played on a DVD player to play for 999 hours if you wish.