battery size issue

Beres my problem, my project needs to fit within a specific space constraint, however the power requirements i need also require me to use battery that is to big for said size constraint. I have some ideas for this such as using a smaller battery and using a sleep mode to help with power consumption and/or a custom shaped battery(although this is a little more expensive) does anyone know of any other methods I can use?

I think a list of hardware, code and a schematic may be a little more useful?

What battery chemistry are you considering?
Are any PCBs using all surface mount?
Do have SMD parts on both sides of the board to shrink it down?
Can you use smaller SMD parts?

Are you working with an electrical engineer?

Have a look at Nick Gammon's power saving tutorial

...R