How to view MTU size in Wireshark

Can please someone explain me on the below screenshot why host 192.168.1.200 on packet 9 sends an ACK for the packets 6 - 7 - 8? The total packet size does not correspond to the window size agreed. If I understood it correctly the window size is the amount o data that a device can send without acknowledging.

The window size is the maximum amount of unacknowledged data that can be outstanding in a socket; however, there is no requirement to fill this window before ACK-ing. In fact, most low-latency connections do not fill the window because stations acknowledge data so quickly. What you are seeing is normal, there is no problem.

This post has a good explanation for what you're seeing.

What you see is normal behavior. Both sides send a small window size and neither announce that they are able to use window scaling. So in the next step they both pull up their TCP window to 64k to allow the other node to send more than just a few packets in case that there is a lot of data to be transferred.

It's quite normal to see that kind of session behavior - as soon as both know that the connection is established and what the parameters are they simply adjust their window.

And,

It is to conserve memory for connection that are not guaranteed to be established yet. The OS/TCP stack needs to reserve as much memory for incoming bytes as promised in the window size (which is essentially just a buffer), so if you waste too much memory on connections from the start you might run out of memory.