I would say that you are probably out of luck. Electric tankless units require a large amount of current (Amps). So much that most houses that want to add one will require an upgrade on thier service (how much current capacity that they can pull off the grid). Being in a condo, it is likely that your current service is smaller than a typical house.
1. Go to your panel and see what is says on the main breaker. It should be a number like "150A", "200A", etc. That will tell us what your total capacity is. Given that you don't have gas, I assume that you are on electric heat (baseboards, heat pump, etc.) and probably have AC too.
2. Even if you have enough capacity at the panel, you'll need a much heavier wire from the panel to the WH. The cost would be difficult to say as it depends on access to where the wire needs to be run. Likely, you would have to open up some walls to run the wire and then repair them.
3. If a service upgrade is needed, I'm not sure what will be involved since it is a condo. Likely this would need to be approved from the condo board as you'll need work done in common areas to run the heavier wire to your panel.
You are looking at a significant cost to gain a couple sq ft. I would probably just stay with what you have. However, if you still think it is something you want to look into, see what your panel rating is and list all your major users of electricity (dryer, stove, electric heating/cooling, etc.). This will tell us if the panel/service needs to be upgraded (it almost certainly does). We can go from there.