When I have seven cards in my hand, can I discard only one with "Sightseer"?
No, you cannot. You must discard until your hand has 4 or fewer cards when using Sightseer.
Source: TPCi Rules Team (2018-12-06)

Relevant cards

Note: All applicable cards may not be listed below.
SM8_189
SM8_189A
SM8_189A