Hi @Nandini -- Cam from the Intercom Support Engineering team here 

When you say “Chat widget” are you referring to the window the Messenger appears in when it is ‘opened’? Or are you perhaps referring to the Launcher icon for the Messenger - this one?

At present it’s not possible to change or customise the size of the window Messenger appears in when it is opened. However, it certainly is possible to customise the appearance of the Launcher icon by creating a Custom Launcher  This guide walks you through how to set this up and includes links to the relevant spots in our Dev Docs for more details. 

Let me know how you go and if you have any follow-up question on this