For sequential search, the best case occurs when:

The search target is near the front of the array
  • The array is small
  • The algorithm is implemented efficiently
  • The search key is small

It is a serious misunderstanding of algorithm analysis to think that best case is related to input size. The array being small has nothing to do with what is the best case input (of a given size).

Implementation of an algorithm (as a program) has nothing to do with the algorithm's best case.

Since sequential search can be used on an unsorted array, the value for the search key (large or small) might have nothing to do with where the record with that key will be in the array.