Can't you type the Japanese characters? There are difference between writing in roma-ji and in hiragana dipending on the textbooks and web pages. Anyway, adjective for BIG is 大きい(おおきい ookii), not おうきい(oukii), and it's i-adjective. 大きかった(おおきかった ookikatta) is the past form. We also have na-adjectove, for example, 元気な(げんきな genkina) and its past form is 元気だった(げんきだった genkidatta). Please take a look at this page: http://www.guidetojapanese.org/learn/grammar/adjectives I hope this helps you.

it's a past tense i-adjective. これぐらい大きかったですか？means was it about this big? na-adjectives are conjugated like だった or でした in past tense.

Simply because Japanese language has sentence patterns that we need to follow to have a correct grammar. -katta (かった) is somewhat a suffix that you add to an adjective when it is used in past tense. おおきかった is past positive adjective. if it is past negative adjective, it will become おおきくなかった (ookikunakatta). but if you want to use simple negative adjective, it will be like this おおきくない (ookikunai). There are two kinds of adjective namely the "na" adjective and "i" adjective. From what I learned in class おおきい is considered as one of the "i" adjective. --But I've read somewhere that おおきい ookii (big) and ちいさい chiisai (small) are a bit two of the exceptions that can be used as "na" adjective.-- (Not so sure about the later part though) hehehe Still learning as well so i hope it helps.