Given that you can use dried maror for the seder (Mishna Pesachim 2:6), and assuming that horseradish is actually maror, what is the minimum shiur of dried horseradish (e.g. horseradish powder)? Is it a k'zayit of the powder, or the amount of powder from a k'zayit of horseradish?
The Talmud (Shabbat 91a) discusses the minimum measurement of food that must be carried between domains on Shabbat in order to be obligated in punishment. The minimum shiur depends on what the intended use of the object is. A kegrogeret (the size of a dried fig, which is greater than or equal to a kezayit) is the minimum amount of food when the food is intended for eating. When it is intended for planting, even a minute amount is sufficient.
The gemara there asks a number of questions regarding someone who transferred a dried fig between domains, but while he was carrying it the fig shrunk and/or grew in size, and/or the carrier's intended use changed. (Fun stuff!) The gemara notes in its discussion that when the fig shrinks in size, it loses its status as a "שיעור אכילה" -- a sufficient measurement for eating purposes.
So it would seem that the relevant factor when determining eating-related measurements is the current size.
The Talmud (Menachot 54a) suggest a machloket tannaim regarding how we measure the kometz of a minchat chotea that got wet with water, thereby swelling. (A minchat chotea was usually brought dry per Vayikra 5:11.) One opinion says we go after the way it is now, while the other opinion says that we view it as it was before the water was added. The gemara spends a few pages debating these two sides but seems to end up rejecting the latter opinion. A ruling brought up in the discussions there is brought in the Rambam Tum'at Ochlin 4:6:
כזית חלב ודם ונותר ופיגול שהניחן בחמה ונתמעטו אין חייבין עליהן כרת, הניחן בגשמים ונתפחו חזרו לכמות שהיו, בין לטומאה חמורה בין לטומאה קלה בין לאיסור
A kezyit of forbidden fats, blood, old sacrificial meat, or mis-offered sacrificial meat [all of which are forbidded with Karet] which was left in the sun and shrunk, one is not obligate in Karet for eating them; if he subsequently left them in the rain and they swelled up again to their original size, they return to their original state in regards their ritual impurity upon contact and their Karet upon consumption.
So we see that when something is dried below its minimum shiur, it doesn't qualify as eating/touching the original shiur of food. However, it can be reconstituted.
Thus we see again that the relevant factor when determining eating-related measurements is the current size.