Hello  Bikram Karki,

You can follow below steps for adding custom height and width on sheet:

1)Prepare your custom filename.less file in /custom_app/static/src/css path.  Put this code in your customfilename.less

.o_form_view{ 
    .o_form_sheet_bg .o_form_sheet{ 
        margin: @padding-base-horizontal; 
        max-width: none; 
        width: auto; 
    } 
    .oe_chatter{ 
        max-width: none; 
    } 
} 

<template id="assets_backend" name="web_sheet_height_width" inherit_id="web.assets_backend"> 
            <xpath expr="." position="inside"> 
                <link rel="stylesheet" href="/custom_app/static/src/css/customfilename.less"/> 
            </xpath> 
</template> 

“data” : [ “view/viewfilename.xml” ,  ]