Optimum aspect ratio for storing data in Block RAMs
No, you have misunderstood the concept of "aspect ratio".

Each chip has a fixed number of block RAMs in it; you can find this number in the datasheet.

Each of those block RAMs holds 18K bits, and can be configured to access those bits in a number of different ways, from 16K words of 1 bit each, to 512 words of 36 bits each. In the first case, you can read or write one bit at a time; in the latter case, you can read or write 36 bits at a time.

For your filter coefficients, you'll probably want to configure the block RAM as 1024 words of 18 bits each. You'll be able to have one filter for each block RAM in your device, and you'll be "wasting" most of the capacity of each block RAM.

There are a few tricks you can try to support more than one filter per block RAM:


If the addressing for all of the filters runs in lock-step, you could configure the block RAM as 512 words of 36 bits, and then split the 36-bit data bus into two 18-bit busses, sending the data to two separate filters.
If you don't otherwise need the dual-port feature of the block RAMs, you can use the second port to feed a second filter. Tie the address MSB low on one port and high on the other.
If both of the above conditions apply, you could conceivably use both techniques together to hold coefficients for four filters in each block RAM.