'File size support up to':
This option gives the ability to open bigger files - up to whatever value you choose in this field (when compared to 'mpTrim-freeware'). For example, if 'mpTrim-freeware' gives you an error 'file too big (418%)', choosing 500% will allow to open this file in 'mpTrim personal edition'.
'Fix missing CRCs':
MP3s have the ability to have a CRC (checksum) protection field. That allows the decoder to check the file on the fly and mute (or lower) the output if it detects corruption.
Not all MP3s have CRCs. It's a choice you have when you encode an MP3. But some encoders do not calculate the CRCs anyway (they leave the fields at 0!). Such encoders are usually based on the ISO MPEG original source code. You may want to fix these CRCs, even though MP3 players usually ignore the blank CRCs so that these MP3s play anyway, but without being protected.
This option gives the ability to re-calculate the blank CRCs, to restore the protection they were supposed to provide.
'Re-create seek information if missing (VBR)':
VBR stands for 'variable bit rate' and MP3s encoded using this method are supposed to have a constant quality - as opposed to CBR (constant bit rate) where the quality may vary because the bit rate is fixed. Some MP3s are VBR, some are CBR, it's a choice you have when you encode an MP3.
Seek information is useful for VBR files. Seek information isn't part of the MPEG standard and doesn't affect the music quality. But players time display will be confused if it's not present, and seek will be inaccurate.
This option gives the ability to re-calculate the seek information header if it is missing from VBR MP3s.