The issue here is the definition of 'Search and Rescue'.
Many countries already have so called 'SAR' fixed wings that in reality can only search for survivors and perhaps drop a draft or stores to them. The 'Rescue' will always need a rotary wing asset.
To answer your question, a UAV could replace the fixed wing portion to search for the survivor. This could then be paired with a rotary asset to then perform the rescue.
The size of the UAV then depends on the size of the search area, and budget of the operator.