As per the previous post I agree with suggestions.
I would aim for a single AA cell as they can have a good energy density. One cell (side ways or horizontal should not be too bulky).
Then "important bit" get a modern efficient switch mode circuit design chip, they are very compact and efficient to drive the LED at a constant current.
This can then work and discharge the battery to about 0.8V getting the most energy out before easily and cheaply replacing.
LED selection is personal, you need to try a few type and investigate the white colours, beam angle, over spill, (hole in the center of the beam?) etc., and may find a few smaller leds a better option. Personally I don't like the ones that give what I describe as a blue' white!
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I meant to add a few links, but look at things like ZXSC380 for the most integrated (or ZXSC300), but there are many others such as OM5448.
PA4TIM's solution can also be investigated but it is less integrated and spec'd, but may be equally suitable for you.