Is it possible to reduce the map file size without deleting blocks? I have a map which is 6.5MB and it needs to be 6 or lower so i can upload it to trackmania exchange. I already tried to compute shadows with the lowest quality but it still doesn't work. Is there any other way to do it ?

Make sure if it's a duo map that you're not doubling up on items. If you and your duo partner have your items installed to different places, you can inadvertently be embedding them twice.

Usually items are the primary cause of having a large map file.

The other can be ghosts. If you have some MediaTracker work on the map and many ghosts, they can inflate the file size.

The primary reason for the 6 MB restriction is because this is the current size restriction for rooms and servers in-game. However, TMX supporters can bypass the site restriction and upload tracks up to 24 MBs in size. You'd still face the size restriction for servers in-game though.