Hi @jatin.devaiya. Unfortunately, there isn't a way to make this happen automatically every time you change the sheet size. Within the Inventor API, there are several 'events' that can be 'monitored' or listened for, so you can do something in response to them, but changing sheet size is not one of them. So whatever amount of automation you may want to create to help with this situation, there will always need to be a manual interaction on your part to change the default text style. The active default text style, when working in a drawing, is defined within the active 'Standard' style, and active 'Object Defaults' settings within that standard style, relating to objects that include text. I would recommend that you create one or more 'Standard' styles for use with each sheet size, in the Styles & Standards Editor dialog (on the Manage tab). Within each 'Standard' style, you can have a sets of other types of styles, and you can specify an 'Object Defaults' type of style to use. Within that 'Object Defaults' style, you can specify which individual style is to be used in the many different situations, while that main 'Standard' style is active.
Then once all that is set-up, you may need to save each new style (of every time) that you have created to the 'Style Library'. That way all these new styles will be available to other documents. Then you may want to update or create any Template drawing files with these new styles, then re-save them. Then when you open a drawing and you want to change to a different 'Standard' style to use, go to the Tools tab > Document Settings > Standard tab, and choose a different Standard from the drop-down list.
Wesley Crihfield

(Not an Autodesk Employee)