Hi,

Try giving style for the field as inline for the tree view of one2many field.

eg: 

<field name="name" width="100px"/>.

Hope it helps

Please goto one2many field tree and add custom class and define CSS, and use data-name of field to mapp 

I have done like this is . 

    custom_size

.custom_size th.o_column_sortable[data-name="name"]{
 width:300px !important;
 max-width:300px !important;
}

i have use this for name column  or name field 

After a lot of searching I found a solution...I put it in my comments

I don't think it is possible to simply set the width in tree view. I've tried but I couldn't.