Which BLX?
Just check the amps of you´re system, take a calculator and choose the Lipo.
If you´re car takes 120A and you prefer an 5500mA Lipo (120 : 5,5 = 21,82... You need more than 21C continuous.
I use 3S 5500mA and 45C for the BLX 200 in the Nero Big Rock.
By the way, in the manual of the car and also outside the box is also an recommendation from Arrma.
Regards,
lonee