"I want a smaller sandwich."
अनुवाद:मुझे इससे छोटा सैंडविच चाहिए।
छोटा = small
मुझे छोटा सैंडविच चाहिए = I want a small sandwich.
The word "smaller" shows that you are comparing two things. So you need a word like इससे or उससे in this sentence.
But where is the other thing in the English version. When it is implicit in English, why it can't be so in Hindi. It may be because there is no comparative word for 'smaller' in present days Hindi. In old usage there was 'लघुत्तर' for 'smaller'. A'ha, but we can still use 'अधिक छोटा' for 'smaller' :)
Therefore, "मुझे अधिक छोटा सैंडविच चाहिए।" should be an acceptable translation for "I want a smaller sandwich." :)