So far the easiest way is to modify the ion-modal-view tag css.
<ion-modal-view style="width: 90%; height: 90%;">
You might also want to play with the top, right, left and bottom property as well to centre the modal on screen.
For e.g if the width is 90% and height 90%, it should be 5% from all sides to center it
<ion-modal-view style="width: 90%; height: 90%; top: 5%; left: 5%; right: 5%; bottom: 5%;">
If you don’t want to add it as inline, then you can make it into a class.