You should always load to a variable. Then it is a lot easier to see where your variables come from. For this case I expect you need to use the size function. (I had to guess the variable name and this loop is overwriting the results)
for i_trial = i_probe_trials' % for all trials of interest
S=load(video_trial{i_video_trial}(1:end-2)); % load video data
dlcmat=S.dlcmat;
nframes = size(dlcmat,1);
traj_y = dlcmat(:,(marker_num_plot-1)*3+2); % get trace from trial
end
