Like a Movie Clip, ThumbGrid can be resized to any size you want. Make it a short, tall, skinny, wide, square, or somewhere in between -- ThumbGrid will adjust its interior layout to match the dimensions you enter.
To change the size of ThumbGrid, click on an instance of the component on the Stage of your FLA, open the Properties panel (Window > Properties in top menu), then enter your custom width and height in the fields provided. For manual sizing, select Modify > Transform > Free Transform in the top menu, then drag the edges of ThumbGrid's bounding box to your new dimensions. You'll notice as you change the dimensions of the component that the size and number of interior thumbnails (shown in the component's live preview, see right) change along with it. With a component size in place, we can then adjust the size and display of its interior thumbnails.
ThumbGrid offers two ways to size your thumbnails: "Fluid" and "Fixed".
"Fluid" is the default thumbnail sizing option. It behaves similarly to a table element in HTML, in that a screen's grid is subdivided into rows and columns and each "cell" is automatically sized for you.
Changing the size of thumbnails when using the Fluid sizing option is easy. Simply change the overall width/height of the component itself (see above), or raise/lower the values assigned to Fluid Rows, Fluid Columns, Spacing and Padding. Each change will create more/less interior space for thumbnails, with the net result being a completely uniform grid appearance.
"Fixed" is an alternate sizing option that allows you to enter the exact thumbnail width and height you want. When used, ThumbGrid will ignore the values assigned to Fluid Rows and Fluid Columns (which are used by the Fluid sizing option) and will instead create as many thumbnail elements as it can within the bounds of the component using the assigned the values assigned to Fixed Size.
When using Fixed sizing, Spacing is used to calculate the minimum amount of space allowed between each thumbnail. So if you enter Fixed Size dimensions that aren't horizontally/vertically uniform in relation to the exterior dimensions of the component, the amount of spacing will always be the value of Spacing *plus* however many pixels are necessary in order to create a vertically/horizontally centered grid within the component.
Next up? Let's talk about thumbnail direction and order in Layout.