You can easily resize the buttons to any size. Just click on them, so you get the handles, then drag out any away from the center on a corner to eyeball the ending size. Once you get one the way you like you can duplicate the others by looking at its X Y values or just type in whatever values you want at the bottom of the workspace or if you want a shortcut use the sizing tool. Click on one thumbnail, then while holding down the Ctrl key include the others in a group and then size as a single unit while they all are selected. I don't like that method because its hader to get them the size you want or in exactly the right position.

Another I thing I did almost immediately when I first started to use DVD-A was change the grid spacing to generate a pattern that matches the typical size of thumbnails I use. That way you can quickly use the grid system to line things up pretty quick. To do that, options/grid settings, play with the values and color of the grid to get what you want then check the 'start all projects with this setting' box.

Tip:

By default the thumbnails when you start out a project and drop one on the work area have a size of 158.4 by 115.2. To make the grid system line up nicely for that size I changed the Grid spacing to 27.2 on the width and 27.2 on the height with no offsets This yields 24 grids across.

Now that gives a nice spacing for six thumbnails of good size on one page. Line up the middle thumbnail so its center handle when selected is at the middle of the 12th cell across. Position a thumbnails at either side with its margin lining up at the third grid cell boundry on either side when its first dropped on the workspace with the orange border. Don't use the bounding box to line up. Repeat and you can get a second row. Adjust as needed to have more or less thumbnails.