Is it permissible to seek treatment from a female doctor that requires exposure of the major 'awrah to treat aspermia and small penis size, knowing that the questioner is about to get married and has not informed his fiancée of this?

The basic principle is that it is impermissible for a woman to treat men due to the looking, touching, mixing, and exposure of 'awrah that this entails. These matters are a means to evil and corruption. Therefore, one must seek a male doctor, whether Muslim or non-Muslim, as the latter is less corrupting than being treated by a woman. It is stipulated that there should be no man available who can provide the treatment. As for your condition, it appears to be something that must be disclosed, because a man's inability to ejaculate prevents a woman from achieving complete pleasure. Consequently, she must be informed. If she agrees, then there is no harm. If she does not agree, she has the right to annul the marriage.