Hi,

The class which defines the chatter when it is in the side is "o_FormRenderer_chatterContainer oe_chatter o-aside"
So, you just need to add it in css. Either you can create a new css file in web.assets_backend, or in addons add the below code:

.o_FormRenderer_chatterContainer.oe_chatter.o-aside 

{
    max-width: 300px; // The width in pixels

}

Hope it helps