How to change the size of item-avatar in ionic?

In Ionic v4 you only need to specify the max's.

ion-avatar {
    max-width: 25px;
    max-height: 25px;
}

With ionic 3 I found the other answers to not work when I put them into my page's SCSS file, this instead worked for me, and is a mild modification of Vairav's answer.

ion-avatar img  {
width:100% !important;
height : 100% !important;
max-width: 100px !important;  //any size
max-height: 100px !important; //any size
}

The item avatar has a default max-width and max-height property. You can find it in the ionic.css file. You can either change it over there or in your CSS just add:

.item-avatar  {     
width:100% !important;  
height : 100% !important;  
max-width: 100px !important;  //any size
max-height: 100px !important; //any size 
}

Best solution I found for Ionic 4 :

ion-avatar { 
    width: 100px;  
    height: 100px;  
}