How Big of a Piece to Separate

Nowadays, we don't specifically take off 1/48; any amount suffices. However, the minhag is to take off a kezayis (רמ"א סי' שכ"ב ס"ה ) – half a beitza (שערי תורת הבית סי' י' פ"א אות י"ב ) – even from a large quantity of dough.