It depends on each patient, their particular conditions and as far as the surgeon can go, but speaking in a range where the technique does not involve a high risk, we speak of between 1 and 2 cm in diameter at most. Going beyond these figures increases the risk.
You can't say that you don't notice anything at all when is erected, but we can say that you really notice it when is in resting state. You have to think that if the patient's erection is of medium or large size, once the penis gains length the transferred fat is distributed evenly. Therefore, those people are seeking a thicker erection.
If the techniques of penis thickening have to be treated cautiously and always going to Centers that have a lot of experience, the techniques of glans thickening on the other hand we totally discourage them. They are based on the controlled inoculation of synthetic material into the glans, which can lead to a number of problems. The volume of the glans is indeed increased, but it rarely leads to a natural and satisfactory aesthetic result. As an added counter-indication, it numbs the nerve endings in this important area of the penis.
There is no simple and general answer, since each case is a world and an operation of this type badly performed can generate a multitude of unique and special problems. The ideal would be to go to a center specializing in reconstructive phalloplasty as is our case and thoroughly review which have been the errors of planning or execution to assess whether they can be retouched and improved (something very rare, the truth) or if as usual we choose to return (for health and safety) to the state prior to surgery.