Objection have to involve 'material planning reasons' to be valid. nd even then the Planning Officer will consider the scale of the impact rather than just refusing outright.
The main material consideration will be the Council's own planning policies which will be set out in their local plan (available on the Council website). Polices will usually require extensions to be in keeping with the character and appearance of the house, to be of an appropriate scale and to not materially impact on neighbours' amenities.
SO, yes the Planning Officer will consider the size of your extension - is it in keeping with the scale of your property, does it result in an over development of the site etc, but not whether you actually need it as such.
Light again is a material consideration - if it is cutting out a significant proportion of light to a primary room in your neighbour's house there may be an issue. IF the room affected is a kitchen/bathroom etc then any impact will have less weight.
THe size of the extension may also have an overpowering impact on your neighbour, or appear visually dominant, but from what you have said, I would think that unlikely unless the neighbouring property is not on the same building line as your property.
IF oyu submitted the application yourself, you could ring the Planning Officer now and ask if they had any views on the likely outcome, and whether there where any material objections. If you had someone else submit the application, they would need to ring.