Object Styles are usually created for frames (graphic, type or undefined). Create a frame using the attributes you want to include. Choose the Object Style panel (if not open, choose Window. > Styles > Object Styles), from the panel menu, choose New Object Style.
Can you save or export an object style so that it can be used in another document? As im trying to retain the same sizing of an object within another file
In your other document, choose the Object Styles panel and from its panel menu, choose Load Object Styles. Navigate to the document that has the object style(s) you need and select them and they will be imported.