You already have much of the information you need, particularly the length of the vector and the total time.
Assuming a constant sampling interval, you can calculate these:
Lv = 87238; % Data Vector Length (samples)
Tv = 300; % Data Vector Length (sec)
Ts = Tv/Lv; % Sampling Interval (sec/sample)
t = linspace(0, 1, Lv)*Ts; % Time Vector (sec)
Fs = 1/Ts; % Sampling Frequency (Hz)
Fn = Fs/2; % Nyquist Frequency (Hz)
You have to choose the cutoff frequency.
There are several ways to design filters in MATLAB, including dfilt and designfilt, however the IIR filter design procedure I usually use may be easier. It is here: How to design a lowpass filter for ocean wave data in Matlab?
