As a very rough compleatly made up guess. Counting ground troops for imperials I would say less than 100 considering most were in transports and didnt enter the base till most of the rebels evacuated. For rebels I would say 800-1000

I'm not exactly sure how many but if you wanted to do that large of a battle you try something a little unconventional that I have worked a little on developing.

Instead of playing one piece representing one individual on a 1x1 inch square, you could do it as one character equals a battalion and play on a large tiled floor since most tiles are 1x1 foot. You could then add some larger terrain like visible 3D building and have a huge battle on a large scale with very few pieces. all you would have to do for stats is multiply the HP by 10 and I think it could be realistic. This is obviously not exactly to scale but it should make an interesting battle when SWM maps limit you in mass battles

I have been considering doing this in our church's fellowship hall since it is so huge and is completed tiled. I have yet to test it though so maybe somebody could playtest the idea and find out if the scale works. I would definitely take a lot of time to set up and you really could have any interruptions since it would be on the floor. Some one could also make a larger scale map of nearly planetary dimensions that would make it possible to move at the battalion level.

Then again it might not work at all but its just an idea. I always one to expand already complicated enough games.

We can approximate it a variety of ways...
Depends on your references, and the assumptions you make.
We see an establishing pan of the entire imperial fleet nearing hoth; many star destroyers etc. Incredible Cross Sections or D20 rpg system (take your pick of 'authoritative' source) lists the landing craft compliment as being 10 + shuttles per star destroyer. This can alter however, if some of the Imperial 2 class Star Destroyers landed on the surface of hoth (as seen similarly in Ep III)... also, the fleet may not have really had that many 'ice world trained' snowtroopers.
Assuming vader used all available landing craft to land forces on Hoth, and that all forces manning the landing vehicles as outlined above are combatants, you'd be looking at around 10 000 Imperials total (around 6 000 from the fleet, 4 000 '501st' from the Executor). Rebels seemed to be around 2 000 total, with around 1 800 evacuating (interesting to note that its only the leadership of the alliance who are at the secret Hoth).
Id say the numbers actually on the ground during the battle of hoth would be close to 8000 imps vs 500-1000 rebels. A daunting prospect, made worse by the firepower/air superiority factors.
Also, the idea that the imps attacked en masse from a single vector is stupid... I'd imagine that Echo Base was completely surrounded, and that the launch tunnel that the falcon flew from was an unknown concealed tunnel.

So, long story short, to make scale recreations, you could go with 8:1/16:1 in favor of empire. Im sure at some places of the battle, that outnumber ratio would have been higher, such as in the base itself moments before the falcon escapes.