listbox and Combobox ROWSOURCE support more 256 characters strings,
but when you read the value, VFP it returns only the first 256:

clear
x=CREATEOBJECT("listbox")
x.ROWSOURCE=REPLICATE('x',255)+'yz'
? x.baseclass,LEN(x.ROWSOURCE),RIGHT(x.ROWSOURCE,1)
x=CREATEOBJECT("combobox")
x.ROWSOURCE=REPLICATE('x',255)+'yz'
? x.baseclass,LEN(x.ROWSOURCE),RIGHT(x.ROWSOURCE,1)