A standard twin fitted sheet will fit the mattress on an Ikea pop-up trundle bed. Most trundle beds, pop-up or pull-out, are a standard twin size. Any twin mattress will fit a trundle bed, and mattresses are typically sold separately.
Twin mattresses vary in thickness, but most twin sheets to their elastic edges accommodate a wide range of mattress depths. Twin fitted sheets stretch to fit a thick mattress or can be tucked under a thinner one. Ikea's mattresses vary in thickness from under 4 inches to just over 10 inches. The twin sheets are advertised as fitting mattresses up to 10 inches in thickness.