what was the issue between small and large states at the convention?

Answer 1

I assume you mean the Constitutional Convention.

It was about representation in Congress. The small population states wanted equal representation by state (and got the Senate) and the large population states wanted representation by population (and got the House). The small states also got the electoral college, which favors the smaller population states.

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The Great Compromise
Roger Sherman was well liked in the convention, and he had the respect of many of the other members. Earlier on June 11, Sherman proposed: "That the proportion of suffrage in the 1st. branch should be according to the respective numbers of free inhabitants; and that in the second branch or Senate, each State should have one vote and no more." This was largely disregarded as too radical at first. Later when neither side would give into the other, they made a compromise, known as The Great Compromise following Sherman's plan of having the House of Representatives be based on population and in the Senate each state would get an equal amount of Senators. This was also known as the Connecticut Compromise. (From Wikipedia)

Answer 2

Size