I found the problem... enough simple !
taille = taille -2;
Sizes can't be negatives, so I corrected the problem :
var taille = 90;
var tailleMini = 15;
function motTaille(mot){
if(taille < tailleMini)
return {text: mot, size: tailleMini};
else{
taille = taille -2;
return {text: mot, size: taille};
}
}