Hello Ajay.
The Header Length is a 4 bit field. That means that it can represent numbers from 0 to 15. The minimum number that the field can have however is 5. The resulting header length is calculated with the following formula:
Length = Header Length * 32 bits
If the value of the Header Length field is the minimum, that is 5, then:
Length = 5 * 32 bits = 160 bits = 20 bytes
If the value of the header length field is the maximum, that is 15, then:
Length = 15 * 32 bits = 480 bits = 60 bytes
The total length is the length of the whole packet. This is a 16 bit field, so it can have numbers between 0 and 65535. These numbers represent bytes directly. The minimum size you can have is 20 bytes and the maximum (theoretically) is 65535 bytes.
Finally, concerning your question about fragmentation, the fragment offset indicates where the fragmentation took place in the frame. This is used to help reassemble fragmented frames. In essence, you are correct in your statement.
The MF flag is also used to aid in reassembly of the fragmented frame. If set to 1, it indicates that additional fragments of the frame will follow. The DF flag indicates that the frame should NOT be fragmented. If the MTU is too small, the frame is just dropped.
I hope this has been helpful!
Laz