The size (in terms of scale, not vertex count) of an object has no influence on the performance by itself. However, rendering an object requires fillrate and the larger the objects are, the more fillrate they will consume...if they are visible. If they aren't visible, it doesn't matter. Most mobile gpus (expect for nVidia Tegra ones) are deferred renderers, i.e. the impact on fillrate won't be that huge.