Your sentence(s) can either be written as:
The value is estimated by correcting the image for a bead size of
25nm. (See above for bead size correction.)
or
The value is estimated by correcting the image for a bead size of 25nm
(see above for bead size correction).
According to New Hart's Rules:
'A complete sentence within brackets is capitalized and ends in a full
point unless the writer has chosen to place it within another
sentence:
"The discussion continued after dinner. (This was
inevitable.)"
"The discussion continued after dinner (this was
inevitable)."'
If you're going to treat it as two separate sentences, then you need a full point/stop at the end of the previous sentence, and the sentence inside the parentheses must have its own full point/stop ALSO inside the parentheses.