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.)
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
"The discussion continued after dinner. (This was
"The discussion continued after dinner (this was
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.