Here's a code segment where I do what I think you're describing. The images are various sizes, but this displays them in a squared-up grid. The trick (at least for my requirements) is to set the height and minWidth for the container that goes into each cell.
<mx:TileList id="imgTiles" width="100%" height="100%"
paddingTop="2" paddingBottom="2" paddingLeft="2" paddingRight="2"
itemClick="eTilesClick(event)">
<mx:itemRenderer>
<mx:Component>
<mx:VBox horizontalAlign="center"
height="250" minWidth="150"
horizontalScrollPolicy="off" verticalScrollPolicy="off"
borderColor="#D2D2D2" borderThickness="1"
borderStyle="solid">
<mx:Label text="{data.imageCaption}" height="15" fontSize="10" fontWeight="bold"/>
<mx:Image source="{data.thumbnailUrl}" width="100%"/>
</mx:VBox>
</mx:Component>
</mx:itemRenderer>
</mx:TileList>