Pre-chat form is not available out of the box, other customers typically achieve the equivalent function via qualification Bot. You could build your own form and then trigger Messenger (passing form fields as custom attributes).
Emojis can be typed via device keyboard, in both directions. What do you mean by chat history?
Logo & Branding available via Homescreen: customers can upload their logo via Admin UI, no client-side coding needed.
Our native Messenger can be minimized, and position can be defined by Admins (left/right/margin-offsets). What are you trying to solve with changing font-size and window-size? What's the problem statement or use-case?
Thanks for your inputs .
WIth point 4 the usecase is that , the window of web-messenger widget is required to be seen extending almost from bottom to top of the page . Also the font size of the letters used inside the chat should be more .