I think the nice way to decline the offer for the first firm is to wait for getting the offer letter, and send a decline letter to them. I think it is always better to be conservative in the answering the job offer. Do not easily say to accept offer. Say thank you and take the time to think about it. However, you are still not in a bad shape, since you did not make any document to show that you accepted the job offer. It looks okay. Just wait for the offer letter and turn down by a letter formally. Since it is assumed that you review the offer after you get the letter, it can be a good excuse to turn down the offer. I do not think there is any drawback to you.

engineer님, 조언 정말 감사드립니다. 먼저 작은 회사는 처음에 생각할 시간을 달라고 해서 다음 주 월요일까지 답을 주기로 했는데 회사의 파트너가 매일매일 전화하면서 갑자기 오늘까지 답을 달라고 하는 바람에 어리버리하게 그냥 덜컥 yes해 버렸네요.

우선은 두 회사 모두에서 정식 서면이 올 때 까지 기다린 후 훑어보고 결정을 하시면 될 것 같네요. 위에 engineer 분이 조언을 참 잘 해 주셨군요. 구두상으로 답 한건 귀여운? 초년생 실수같습니다. ㅎㅎ