The limit of resolution of a telescope is measured by the angle subtended at its objective by two distant objects which are seen as seperate through the telescope.
Limit of resolution ∝ Diameter (d) Wavelength (λ)
Telescope is used to see distant objects which are generally seen in sunlight. Therefore, we have no control on λ, hence we use object of large aperture (d). Larger the aperture of objective lens, smaller the limit of resolution or greater the resolving power. Also large objective sends greater amount of light in the telescope and so intense images are formed. Also large aperture increases lens abberation.