Selecting the Low Pass FIR Filter, Decimation, DSP
When you are decimating a signal x(n), to x(an) (a>1),in the frequency domain the spectrum expands. This might lead to aliasing of the adjacent spectral bands.So to avoid it we use a "anti aliasing filter".

Now choosing an FIR low pass filter depends on many factors like, order of the filter, attenuation in stop band, range of pass band frequencies, etc..

Till my knowledge, sampling is not at all a point of concern while designing the filter. you only decimate the signal after filtering.

To quote an example, in the base band part of digital communication, at the transmitter side, you upsample the signal and you use a anti imaging filter, to avoid multiple copies of the same spectrum being repeated. where as in the receiver you first filter out the signal and then decimating it for further processing.

Cheers,
phani tej