How do you run a 300mb+ MS SQL .sql file?

From the command prompt, start up sqlcmd:

sqlcmd -S <server> -i C:\<your file here>.sql -o

Just replace with the location of your SQL box and with the name of your script. Don't forget if you're using a SQL instance the syntax is:

Here is the list of all arguments you can pass sqlcmd:

Sqlcmd            [-U login id]          [-P password]
  [-S server]            [-H hostname]          [-E trusted connection]
  [-d use database name] [-l login timeout]     [-t query timeout] 
  [-h headers]           [-s colseparator]      [-w screen width]
  [-a packetsize]        [-e echo input]        [-I Enable Quoted Identifiers]
  [-c cmdend]            [-L[c] list servers[clean output]]
  [-q "cmdline query"]   [-Q "cmdline query" and exit] 
  [-m errorlevel]        [-V severitylevel]     [-W remove trailing spaces]
  [-u unicode output]    [-r[0|1] msgs to stderr]
  [-i inputfile]         [-o outputfile]        [-z new password]
  [-f  | i:[,o:]] [-Z new password and exit] 
  [-k[1|2] remove[replace] control characters]
  [-y variable length type display width]
  [-Y fixed length type display width]
  [-p[1] print statistics[colon format]]
  [-R use client regional setting]
  [-b On error batch abort]
  [-v var = "value"...]  [-A dedicated admin connection]
  [-X[1] disable commands, startup script, enviroment variables [and exit]]
  [-x disable variable substitution]
  [-? show syntax summary]