do it manually, its your best way of setting it up around your player without getting extra HULL hang over.
vec_set(my.min_x,vector(-x,-y,-z));
vec_set(my.max_x,vector(x,y,z));
Here is what i have from an old script i fit it perfectly around my character
vec_set(my.min_x,vector(-10,-20,0));
vec_set(my.max_x,vector(10,20,80));
The my.min z attribute is set to 0 because in med i put the grid at the bottom of my players feet. So know it starts at the feet and goes up 80. 10 in front and 10 in back, 20 on the left and 20 on the right. This is a fairly slim hull you may need to adjust it but when your testing with it just hit F11 twice and take a look at the box, play around with the numbers and notice the difference.
Also its in the help file look it up.
You can also use the BBox setting which sets a fairly large HULL around your entity or you can use POLYGON flag which sets a fairly proportional HULL around the entity but min/max is better if you need to get the HULL just right.