One of the reasons for eating karpas at the seder is so that no bracha need be made on the maror. On the one hand, maror should require its own bracha (even though it is during the bread-meal) being that it is neither part of the meal nor enhancing the meal. On the other hand,since it was on the table at the time the hamotzei is recited and everyone knows it will be eaten during the meal, a separate bracha should not be needed. To alleviate this problem we make a hoadama on karpas and have in mind the maror as well.
However, there is a debate amongst the rishonim as to how much of karpas one may eat. The Vilna Gaon held that one should eat more than a k'zayis of karpas because otherwise it is not considered an achilla. Additionally, less that a k'zayis would not necessitate washing (orchatz). Most other poskim (e.g., Shulchan Aruch 473:6) hold that one should be careful not to eat a k'zayis because eating a k'zayis may require a bracha achrona and if a bracha achrona is made the hoadama will no longer work to include the maror.
As explained in a previous Brachos article, "Time Out for Bracha Rishona," it is not problematic that a bracha rishona was made on karpas and there may be as much as a two hour gap until the maror is eaten because a bracha rishona does not expire from being in effect so long as on had in mind a specific food that he may eat later on.
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