This can be done with the help of CSS but I don't think so it would be a good idea because in dialog the appearing wouldn't be nice if dialog width is less.
You can place this CSS file in a client-lib and make the category of that clientlib to "cq.authoring.dialog". then it will get included in the Touch UI dialog.
so you mean, i have to create clientlib folder under my custom component and have the css file under that client lib folder and add a property called cq.authoring.dialog under clientlib , then css will be reflected in dialog box ?
yes, you need to make a clientlib folder and then place this CSS file under that and add a property called categories with value "cq.authoring.dialog". Please find the below reference image.
I'll suggest you to add granite:class property in your items to add extra class which can handle your item layout otherwise you have to take care to apply it only for specific item.