what was the most amount of land we could of got?
The very most was All of Mexico. Mexico was utterly defeated at the end of the war, all you need is the political will in the USA to get more land. There was a substantial movement in the Congress and the US public to annex Mexico outright as a territory. It was defeated since the anti-slavery movement did not want to give the Southerners even more space to expand slavery into (a mistake on their part, such expansion would have substantially failed since Mexicans were anti-slavery and freesoilers were quite able to outcontest the West to the slaveholders as Kansa and Nebraska showed, if anything Mexican states would have been a substantial help to the Union in the ACW, giving the Confederacy a second front) and the Southerners themselves had second thoughts because their racist hearts worried about eventually having to give US citizenship to indioes and half-bloods. You need to weaken these concerns sufficiently, or have some occur that persuades the Americans that Mexico is going to be a long-term recurrent threat to them (e.g. a much-publicized massacre of US war prisoners).
Alternatively, the Guadalupe-Hidalgo treaty itself was negotiated by an half-rogue US diplomat that outstepped his mandate and tried to mollify the deal to the Mexicans somewhat, US President Polk was rather PO and half-thought to renege it, OTL he eventually thought it was a sufficiently good compromise and endorsed it. Have him do renege the treaty, and send another, official delegation to claim more extensive territorial gains. Maybe not All of Mexico in this case, but a good alternative would have been the 22° Parallel.
Another alternative: Soon after the break away of Texas, there were two other secessionist republics, Rio Grande (1840) and Yucatan, which sought US help for its independence and were fairly pro-US. OTL, such help was lukewarm, and Rio Grande quarreled with Texas over territory, so even Texas did not help them decisively, and Mexico reannexed them swiftly. Have the US Government be more energetic in supporting them, or Rio Grande be more deferent to Texas about the Neuces-Rio Grande claim, so Texas will help them win. Advance to 1845, both Texas and Rio Grande successfully plead for US statehood, which will PO Mexico and precipitate the war. Rio Grande's territory extended down to Tampico, therefore in such a case the most natural thing for US will be to prolong that boundary to the Pacific, which again roughly follows the 22° Parallel. If Yucatan is still independent, they will claim statehood, too, which will mean even more territory for the USA.