Help with a rather specific implementation of FFT Library?
As Edgar has mentioned you are confusing things slightly.

First there is your sampling frequency. According to the Shannon-Nyquist theorem that is at a minimum of 2x the maximum frequency in your signal. So that is 2x100 = 200Hz. That's the number of samples per second in your sample set.

Secondly is your sample size, which is directly related to the FFT bucket size. For every two samples you can get one bucket. The more samples you have the more buckets you have, and the higher the resolution each bucket is. But for FFT to work you need a power-of-two sized sample set. That means 2, 4, 8, 16 etc samples.  So 256 samples would give you 128 buckets. That would be a little under 1Hz per bucket.

To make things cleaner though you can back-apply the last calculation to the first and get them to match. 256 samples at 256 samples per second will give you 128 buckets covering 0-128Hz so 1Hz per bucket.  Double that to 512 samples and you get 256 buckets at 0.5Hz per bucket. Double it again and you get double the resolution again. 

The main speed issue that you will come across is not the time taken to do the FFT, but the time taken to capture your sample set. 256 samples at 256 samples per secod will, of course, take 1 second to capture. 512 samples will be 2 seconds, etc. Far longer than a decent FFT takes to calculate.