Put that table in design view. Select the field that the textbox is bound to. Look at the properties of that field and see if there is a character limit imposed on it. By default, it is 255 but may be less. If the level of 69 or similar isn't set, then it may be being limited by code. In that case, put the form in design view, select the control and look at the Event tab on the property sheet. It could be one of a few events where this might be the limitation is.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.