The board is 28x58mm, pretty small and should fit in a lot of engines.
The battery size is always the issue for those going battery-powered, the NiMh packs I use are approx 1x1x5 inches, the LiPo packs are a bit smaller. Of course you have to be able to charge them which requires access to the battery or a charging jack. I saw a round charging jack mounted on the rear wall of the tender and was very inconspicuous and easy to get to.
It will also require an on/off switch.
Of course you could always charge from a piece of track that was setup with power.
What kind of power requirements are you finding with your battery power. I wanna start planning for my locomotive surgery.
When I was still running DCS and TMCC/Legacy, I measured the current on all my engines, steam and diesel. All were under 1 amp, except my Williams brass 4-8-4 J which measured 1.75 amps pulling a 5-car train. The Js motor had a bent shaft and I replaced the motor but didn't re-measure.
I haven't measured any of the steamers (single motor) since I switched to battery power, but so far I've been using either a 9.6volt 2200Mah NiMh or 11.1volt 2000Mah LiPo battery pack and have had no problems.
Here's my "Disclaimer" before anyone jumps...I only have the DC can motor, headlight, backup light consuming power from the battery pack, no smoke, or sound (well, I did install a MyLocoSound sound package in a Railking 2-8-0 and it worked fine). I don't use smoke, removed the units when I was running command control.
I will say this, the Williams brass J weighs 14 pounds. I have pulled a dozen passenger cars with it and had no problems. I usually run a 5-car passenger train and at 2lbs each, that's 24 total pounds being pulled using a 11.1volt 200Mah LiPo battery pack. I normally get over 2 hours run time before recharging. Remember, the engine is the only thing using power from the battery pack. I currently have no lights in any of the passenger cars (and no wiring to the layout), passenger car lighting could be done using a battery pack/LED system, or could even be powered thru the rails. If I had kept my 3-rail system I could have powered the lights in the passenger cars easily, but with 2-rails (I pulled the middle rail out) I would need to isolate the wheels in order to use track power for lighting.
The RCS system I'm using has polyswitches built in to cutoff power if current is too high.
When I first started this everyone was saying it couldn't be done. I went ahead anyway and converted one of my RailKing 2-8-0s and have not looked back. Every day I read the problems folks are having with command control signals, wiring, losing comms with their engines, etc, etc, and know I made the right choice for myself (and I was not having any real issues with command control).
I sometimes miss having the steam chuff, bell, and whistle, but I feel that'll soon be OBE (overcome by events). I can't wait for BlueRail trains (even MRC has their Genie system on their site) to offer a board for O scale with enough amperage to suit everyone's wants. I think 2amps will do for a lot of small engines and uses (like my Railking Imperial 0-6-0, used for switching), but 3-6 amps would cover everything I can think of. The RCS system I'm using is a 3amp setup, using the Deltang Rx65 receiver/ESC combo board.
I think I'll watch the video Joe posted and give that a try.
I hope this helps, if not post again and I'll give it another try!