What is the maximal frequency resolution for Matlab's STFT implementation spectrogram()?

Matlab's spectrogram() function calculates the STFT of a signal. It describes its NFFT argument as follows:

S = SPECTROGRAM(X,WINDOW,NOVERLAP,NFFT) specifies the number of frequency points used to calculate the discrete Fourier transforms. If NFFT is not specified, the default NFFT is used.

Am I correct in that NFFT is a trade-off only between frequency resolution and number of computations? For my offline work, there's no need to save cycles. Is there any maximum limit for NFFT, imposed e.g. by spectral leakage, or any other problem that I should know about, or can I set that argument to as high as possible?