There is no MTU in the IP header as defined in RFC 791. What is there is Total Length that refers to the number of bytes in the packet (max 65,535) before fragmentation.

I don't know if Wireshark calculates the size of each captured packet but unless it is an obviously short packet it will be either 1500 or 1492 (IEEE 802.3/802.2) depending upon the source of the capture. Common MTU's are listed in RFC 1191.