Praise be to Allaah.
It is not correct to divide matters, when deciding whether it is permissible to pray istikhaarah regarding them, into the two categories of matters which are subject to rational thinking, in which one should not pray istikhaarah, and matters that are not subject to rational thinking, concerning which one may pray istikhaarah.
Rather what is prescribed in all cases - both major and minor - is to use one's intellect and wisdom, and to study the options and available means. If a person is hesitant about some matter or it is not clear which choice is the correct choice on the basis of shari evidence, if it is a shari matter, or the basis of rational proof or proof based on experience, and so on, according to the nature of the matter in question - if he is uncertain and it is not clear based on whatever evidence is appropriate in that case, then he should delegate his affairs to Allah, asking Him for guidance and declaring his helplessness before Him, and pray istikhaarah, which is a supplication to Allah, asking Him for guidance and success after examining the available options on a rational basis.
Istikhaarah does not cancel out rational thinking or examining one's circumstances; rather it is complementary to that. Islam does not approve of overlooking appropriate measures or relying completely on these measures; rather it is a balance between the two, so that the Islamic character will combine being realistic and being spiritual at the same time.
Hence we can say that istikhaarah is prescribed in all our affairs - so long as they lie within the realm of permissibility - alongside using intellect and studying the available means.
It was narrated that Jaabir ibn Abd-Allah (may Allah be pleased with him) said:
The Prophet (blessings and peace of Allah be upon him) taught us to pray istikhaarah concerning all our affairs as he would teach us a soorah of the Qur'aan.
Narrated by al-Bukhaari, 6382.
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
It includes both important and insignificant matters, because important matters may stem from insignificant matters.
End quote from Fath al-Baari, 11/184
And Allah knows best.