גָּדוֹל יְה gadol ya: גדול is an adjective meaning: "great, large, tall, high, mighty, old, elder, noble, proud". Ya is the way I have written the Holy Name of HaShem, which is yod-hey-vav-hey in the Hebrew. This all translates: "Great (is) G-d". (The "is" has to be supplied, as there is no present form of "to be" in Biblical Hebrew.)
וּמְהֻלָּל מְאֹד; oo m'hoolal meod. You may recognize by now the shoresh: הלל. The verb shoresh is הלל haLAL, is the shoresh for the familiar "hallelujah". It means: "to praise, celebrate or glorify." HOWEVER, the niqqud reflect the pual form, which is passive, and changes the action thus: "to BE praised, to BE celebrated, to BE glorified" by another. מאד meod, means "power, strength, force, with all might, to a high degree, very much, exceedingly" The two words could be translated: "and highly to be praised"
וְלִגְדֻלָּתוֹ, אֵין חֵקֶר. va-leeg-doo-lah TOE, ein CHEqer. Do you see the shoresh גדל in the first word? vav = and, lamed = to, g'dool = greatness, might, power, distinguished, toe = of him: or the entire word in Hebrew is "and to his greatness"
אֵין ein means: "there is not, there is no, has no, there is nothing, there is none, bereft, no/not one"
Finally, חֵקֶר CHEqer means: "searching out, examination, meditation, deliberation, fathomable, a mystery". If we put this all together, we have "And to the greatness of him, there is no fathoming (it)" That's my take on it. Other ways it is translated: "and His greatness is unsearchable" KJV, "his greatness no one can fathom" NIV "no one can measure His greatness" NLT.
Just as a note: I use these two sites extensively along with my Biblia Hebraica Stuttgartensia and Langenscheidt's Pocket Hebrew Dictionary.
http://www.mechon-mamre.org/ click on "parallel Hebrew and English"
http://biblos.com/ click on "interlinear Bible"
להתרות l'hitraot (see ya!)