Open ompt file
create a sample with 128 samples
create a second sample

now mod plug will ask to create a 64 samples long sample
here modplug should remember the last used amount of samples and ask to create a 128 samples long sample as the first one that was created
if the first sample was 512 samples long then it should ask to create a 512 samples long new sample and so on.
but after in example 3 samples you create a 64 samples long sample, so from here onwards mod plug should ask to create 64 long samples.
just always remember the last used setting and suggest it as the next default.