I'm gonna repost my thoughts from the other day.
Right now the demo roster has a 4 by 8 layout on it.
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To keep the same proportions on the CSS and the portraits, you would have to rescale the portraits to be 2/3 of the current height and width, but multiply the number of portraits to be 3/2 their current amounts, making the CSS 6 (4x1.5) by 12 (8x1.5).
To add to this, the button sizes for the portraits currently are 3 buttons wide, but if the portraits scale down to 2/3 their current size, they'll be 2 buttons wide (which is enough for 4 buttons to fit on the portrait provided the buttons are positioned in a zigzag motion)
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Assuming the space is completely filled, this provides 72 slots. We have 64 different character selections available according to the smash bros website (assuming echoes take up a spot on the css, and miis/pokemon trainer both count as one slot on the CSS). This provides us with 7 slots remaining, because one slot is removed for the random button.
This could all change and be mere conjecture, but this is what the signs currently point to with Sakurai stating not to expect many newcomers and many people's expectations with the roster size.
The knockback sending targets instantly to their location stifles combo games
The cloud knockback effect is too exaggerated
and Ice climbers/cloud appear to be completely gutted