Is it excused if there is an obstruction of one square millimeter or slightly more on the foot after ablution and prayer, or generally, an amount the size of a lentil?

Some scholars lean towards pardoning a small barrier that prevents water from reaching the skin, such as a little dirt under the fingernails. This view was affirmed in Al-Ri'ayah al-Kubra and chosen by Sheikh Taqi al-Din, who also extended it to include any small impediment on the body, such as blood or dough. The determination of what is little and what is much is referred back to common understanding among people. It appears that the barrier mentioned in the question falls under the category of what is small and excused, as does the amount of a lentil seed.