Thank you for your quick response! I looked up online and some people from the researchgate are saying different fmri lengths may have different results. Is CONN doing anything specially to avoid such effects during the preprocessing/processing steps? Or do the different scan durations not result in any systematic difference at all? If I only want to analyze the first 165 volumes for each scan, can I select the corresponding scans directly in the setup from the CONN GUI? If not, I found the following commands to remove volumes. Is it correct? Is there any way easier?

idx = [166:210]; 
a = spm_vol('MyFunctionalFile.nii');
spm_file_merge(a(setdiff(1:numel(a),idx)), 'MyNewFunctionalFile.nii');

Thanks again, 
Jianing