Have had all of these
Electric Blue and also White Alleni crays grow to approx 5 inch, some may get to 6 but in my experience that was top length.
They are extremely aggressive
, you only want 1 in a BIG tank (definitely over ten gallons) they will eat your plants, and fish! In fact anything they can put in their claws they eat and as they hunt at night your sleeping fish are a prime target for them.
You'd be better of with Dwarf crays, CPO's which are the small 1 1/2 inch Orange ones.
There is a dwarf species that comes in a blue color (very rare) called Shufeldti...IF you can get a pair of blue ones you can breed the blue color in each generation, but most of the ones on the market are brownish/blue and will NOT breed true.
I raised these last year after buying 3 pairs from a breeder on aquabid, sorry don't remember who it was. All the babies were blue/blue grey and they are very peaceful, can all live happily together in a 10 gallon tank (and did) and I had lots of babies...sold all of them
Orange CPO's need lots of space, so a tank smaller than 30 gallons is going to create a problem...they get aggressive with one another, especially if one is a female (they usually are bigger too) and they will kill a male or another female over territory.
I just picked a pair up and put them in my 30 gallon and only one of them is still alive