How to calculate bandwidth require for OFDM
The channel bandwidth and FFT size alone don't provide enough information to describe the entire structure of the OFDM signal. Recall the following relationship:

$$
\Delta f = \frac{f_s}{N}
$$

where $\Delta f$ is the subcarrier spacing, $f_s$ is the sample rate used at the modulator input, and $N$ is the FFT size. 

In your LTE examples, the two channel bandwidths must use the same subcarrier spacing, as it takes four times as much bandwidth to carry four times the subcarriers (512 versus 128). 

For 802.11a, however, the FFT size is typically 64; with a sample rate of 20 MHz, this yields a subcarrier spacing of 312.5 kHz. Of these 64 potential subcarriers, 12 of them are unused (sometimes referred to as "virtual carriers"). Of the remaining 52 subcarriers, 4 contain pilot tones, so only 48 subcarriers carry information at any given time.

For your notional OFDM system, you can choose the amount of bandwidth required by your system, as required based on your throughput requirements. Choose the sample rate that you will input to the modulator, then choose a convenient FFT size. This defines your number of subcarriers and therefore the subcarrier frequency spacing.