Well, the only thing I can think of without doing a lot of testing myself are one of the following depending on what you are trying to achieve:

1. Use an Orthographic camera to render your sprites.
This will not solve your physical sprite size issue, but it will solve the issue of general lens distortion where items closer to camera seem extra large compared to those behind them.

2. Use a set driven key.
Create a locator and parent it to the camera.
Create a set driven key to drive the SpriteScale x&y to appear the same as the sprites get closer to the locator.

3. A mel script that does the same as the above, but with distance to camera as the constant.

Good luck!