What is safe dose of aspirin for the purpose of blood thinning at onset of tia-like symptoms prior to diagnosis can be confirmed?

325 mg daily: Would start on 325 mg and get to the hospital. A TIA can not be differentiated from a stroke initially - until the symptoms resolve. If symptoms resolve, it is a tia. If they don't, these persistent symptoms represent a stroke - which requires immediate treatment.

81mg: Baby Aspirin (81mg) can be taken. One possible caveat is that sometimes brain bleeds can mimic TIA or stroke. In that case taking Aspirin can result in bleeding complications.