Sprite boundaries

[Boba Fonts]

Since I'm making and intensive use of pixel shaders, I'm facing the sprite boundaries limitations: shaders only affect what's inside the sprite rectangle, and for example a skew shader (I'm using it for a shadow effect) distorts the image bending it until it's truncated because it extends out of the "box". If I want the complete effect I must expand the sprite boundaries with a lot of blank space, maybe 100, 200 blank pixels of additional width on the right and on the left.
I suppose there's no way in the world I can expand the blank space in runtime in the events, since sprites are treated as bitmaps, right?
So I should manually reposition the sprites inside a wider blank space, ok. Then, will this operation make the game heavier? In terms of memory, how much I'm going to lose with sprites five times bigger than before (given the fact the majority of their space is blank space)?
When the whole thing will be zipped the blank space will shrink to almost nothing, but in runtime great amounts of blank pixels count as full bitmap pixels?

[nivram]

With HWa it should not make a differnce.

Marv

[Boba Fonts]

So a cropped bitmap character, say, 80*220 is the same (in terms of performance) of the same sprite but in a 320*220 box with wide blank space? Wow! That's good news.
And in terms of .mfa filesize?

[nivram]

Yes, the file size will be larger.

Marv