>1-Does it take all the files from a PSX cd and forms it to one file?
When creating an ISO a "bit for bit" copy is done. That is, the disc is copied as one unit, not as seperate files. Doing this is sometimes essential as quite a few PSX games include copy protection in the form of errored out sectors. Likewise, if you were to merely copy the files they'd either be corrected or omitted and you'd have a non-functional game. The only caveat is in regard to error correction, as some forms of ISO dumping will include it while others won't. Generally this isn't important enough to worry yourself with though, as it often makes no difference. If you're a glutton for punishment you can go check out the CDR FAQ for more info about error correction.
>How much space will it take, for example if I would make
>Resident Evil 3?
I don't own the game, so I can't speak with any authority, but most ISOs tend to be 600 to 750 MB in size.
>And how long does the process take, to make an Iso?
It depends on the speed of your CD-ROM drive and the amount of copy protection (if any) that's present on the CD. Using a 40x DVD-Rom drive (rated at 18x by CDRWin) it generally takes from 9 to 15 minutes.