How Bert change max sequence length when we do fine tune task?

Suppose we use pretrained model with max sequence length 128。
Now I change the config file, reduce the max sequence length from 128 to 64。
Next, do the fine tune task , such as a smiple classifaction task。
My question is, during the fine tune, will the dimension of model changed ？ Since it has many part relevent to max sequence length.
How does bert solve this problem when we change the max sequence length for fine tune.